I'm not an expert but these are my thoughts:
How big is the yard the kids play in?
The only way I would feel safe is to visually inspect their play area every time they go out to play. Use a broom handle or something long to poke anywhere you think a snake could hide. Keep the grass short so you can see if anything is hiding in the grass.

Know the habits of snakes: when they are likely to be sunning themselves in the sun, when they are likely to be seeking shade under a rock. That their striking distance is only the length of their body. That not every bite has a lethal amount of venom in it.

Teach your kids to never put their hands anywhere where they cannot see what they are touching, for example, to reach behind a rock or to pick up a piece of wood. Teach them what to do if they ever see a snake. Remind them every time they go outside to play.

Have a game plan if anyone is ever bitten. Emergency numbers, etc. Know what to do medically (keep the victim calm, etc)

I do not think that eliminating the wildlife is realistic. But I would remove any sources of water that you can from your yard. The wildlife would come to drink and then the snakes would come to wait for their meal.

I encountered a rattlesnake once when I was hiking with my dog in Griffith Park. My dog was on the trail in front of me. I heard the rattling before I saw the snake. I froze, then I stepped backwards, and I yelled, "Buddy, COME!" God bless her, she did!. We hightailed it back down the trail. If I had been alone without my dog, I would have frozen and ever so slightly inched backwards. But with my dog, I felt I had to get her out of there immediately.
I never felt at ease every again when hiking with her.

Michele: We are also in the country and live with the presence of Rattlers all the time. I also worry because of our small grandchildren. I don't know how little yours are, but even our 3 year old knows to listen for SSSSSSSSSSSS -- AND LEAVE. I don't think there is much you can do to keep the critters away that they feed on because "Nature abhors a vacuum", and so new critters will come to where there is no competition..... and the hunters will follow. One thing I have found that really works: when you see a Rattler gently spray them with the hose... they don't seem to like to get wet and leave; (and you don't have to get close to do this.) After awhile they get the hint and leave.

When you live in wild animal territory you just need to learn to look and listen when outside, and you need to teach your children to Look and Listen also.
I was taught to stand up straight and look ahead, not at the ground, when walking. I've learned to look down and around (corners, gates, bushes, pottery, etc.) as I walk. I can't tell you how many Rattlers I've almost stepped on when I hear the "SSSSSSSSSSSSS -- I'm here!" signal. I just say, "I'm sorry! I didn't see you", and, "Have a nice day", and pass on my way. If said Rattler is in the yard-proper I go get the hose and a gentle spray gets them on there way.

Oh yes, always look before putting your hands or any other part of your body under a dense plant/shrub to do gardening, retrieve a toy, etc. And, if you do get bit: put a cold pack on the bite and call a rescue squad. If it's you, absolutely don't try to drive.

Another wonderful warning: watch out for the Coyotes, especially if you have small animals. They live in those canyons too.

Reptiles need a couple things ...

1) You mentioned their food source.
2) Heat. They are cold blooded and must warm up to be active.
3) Places to hide.
4) Lack of predators such as Hawks and Coatimundi.
5) Access

If you saw the snake out in the open, you likely saw it on a concrete deck or a paving stone or some other area that is naturally warmed by the Sun. You might want to evaluate whether you are providing reptiles a preferred area to Sun themselves or absorb heat from concrete or other solar heat absorbing material.

Also evaluate your yard area to see if you have any areas where they may crawl under objects to take shelter.

Yes there are certain predators that feed on rattle snake, certain Hawks, Eagles and some mammals such as Coatis.

Access is a difficult thing to eliminate. But you might want to evaluate your fencing, etc.

Michele I hate them also and very scared of them. I don't have any advice that already hasn't been given.
I feel the same way, the entire hill they can have except inside my fence!
I have a water feature that does attract them and had a young one a while ago early in the morning, all curled up and sleeping near it. Thank you firemen!
I do have hawks, an owl and coyotes also.
I you think you have mice or rats around plant some rosemary. They don't like that I am told.

Regarding Access ...

Here's some more info.

It occurred to me that the sensory mechanism for snakes, their flickering tongue makes them vulnerable to the possibility of certain irritants and thus ... repellants.

I checked on the web and yes! There are such things as snake repellants. These often contain sulfur and some other oil base, like naphthalene or certain seed oils. Some are not distributable to California however. Check for availability of such compounds. They may work or at least help.

As far as food goes, eliminating mice and rats is something to think about, either by having owls, cats or other rodent predators ... or by repelling the critters snakes feed on.

A combined strategy is likely a better one than just using one tact.

Heres an interesting link ...

Here is a link that might be useful: Homemade Garden Snake Repellant

I just saw a home re-do program on HGTV about this very subject. They re-did the dust-hole-ish backyard by putting in a six foot cedar fence all around the rectangle of new sod, and on the exterior they put a six-foot-wide wire mesh fence, dug two feet down underground and stapled the other four feet up the fence, all around the exterior of the cedar fence.
Cost a lot, but they had a nice green safe backyard for their toddlers.
Carla in Sac

Unfortunately, the snakes are here to stay and there really is no way to eradicate them.

The best thing you can do is to make your yard and home as inhospitable to them as you can. Trim all shrubbery about a foot off the ground so snakes would be easily visible. Remove brush and wood piles. Seal the bottoms of decks or patios and make sure vents are also sealed off. Basically get rid of places that might offer any sort of shade or shelter to the animal and make any that might wander in the yard visible. Don't leave any dogfood outdoors that might attract rodents.

Teach your kids about them and learn all you can about their habits, behaviour, etc. The more you know, the less fearful you will be. Learn to be aware and prepared, rather than afraid.

It would also help to have a plan if one is in the yard. A good snake tong (about $50) and a 5 gallon bucket with a lid makes removal easy. Or a tall trashcan placed on top of the snake will keep it contained until someone else can remove it for you.

wanda

I forgot to mention.....They will soon be going into hibernation for the winter so you shouldn't be seeing too many. Generally they are most active in spring when emerging from hibernation, seeking food and matings. Then again in late summer - fall when the babies are born and they are retreating to dens for the winter.
During the summer, they are generally more nocturnal, seeking shade during the heat of the day and hunting at night, especially hot nights.
Older, more experienced animals often give "dry" bites, and do not inject venom. It takes a lot of energy to manufacture venom and they would rather not waste it unless they truly feel threatened, because that wrecks their chance of catching a meal. The younger ones are like Chicken Little (the sky is falling) and are more likely to inject venom.

Here is a link that might be useful: rattlesnakes

Thanks, all. I know I should just deal with it. But it really has taken the fun out of gardening for me. I don't even want to walk past the patio where the big rattler was sunning himself. He was only 2 ft from the house so it wasn't in the depths of the yard or anything. I haven't let my girls (ages 3 & 5) play there since. I wish I could afford the 6 ft fence and wire-mesh plan described above, or a solid stone wall, or something professionally installed. Our DIY attempts to seal the one fence (b/c rabbits were eating everything that I planted) was a joke. The critters are easily able to squeeze around, under and through all the places the chicken wire doesn't quite cover. The super-strong Santa Ana winds pull down fences here easily if they aren't very strong. Our backyard has a large area against the canyon, on several levels. Plus we share property line with neighbors on either side, one uphill from us and one downhill, who would have to be on board, right? We live in the suburbs of north Orange County.

I have removed all low-growing shrubbery next to the house for visibility. But there are many potted plants, plastic Little Tykes play ground set, 3 big trash cans, outdoor furniture set, firepit, bikes & trikes & toy chest, A/C unit, BBQ, watering cans, so many hiding places to worry about...So no one has ever tried repellents or electronic devices for rodents or snakes? I'm the only one who has freaked out to this level???

My guess is if you have seen one there is a very good chance you will see others over time. Not every day or even every month, but over time, you will likely see more.
+1 for checking for places they can get under and hole up.
I think the best defense though is cats. They busy their days riding you of mice and such, which is good in it's own right and also is getting rid of snake food. And, a really good cat can even attack a small snake and kill it.
Last but not least, make sure you know what a king snake looks like and do NOT harm them. They can actually kill and eat a rattler, thus their name.

King Snakes are a great idea.

But if you buy some, make sure they are native species for your area.

Also, make sure what you think is a big Rattler isn't a big Gopher Snake... another good one. I've found that when Rattlers and Gophers get BIG, 5 FT +++, their patterns look very much alike, so I check their tail and head.... which are nothing alike.

If you want to be absolutely certain, what you really need is snake fencing, wire mesh that is of a certain diameter (so the snake can't get through the mesh holes) and that is buried a foot or so, so that the snake can't get through underneath. Do a google on 'snake fencing' and some sites should come up.

I've lived with rattlers 50+ years. Keep cats to reduce rodents and find snakes. Keep a dog to find snakes. Inform your kids. No snake havens, like wood piles, near house. The only people that know that have been bitten were messing with the snake. I stepped on a rattler barefooted without getting bitten. The snake is way more scared than you are. The risk of a kid being hit by a car in front of the house in the city is way worse than the snake risk in the 'burbs.

Thanks. Very reasonable advice! I think I am calming down as the shock fades a bit :)

Hi Michelle, no, you are not the only one that has been that frightened or upset. A long time ago I posted on here about my having to kill my first rattlesnake, the beejeezus dance I did before and after and how I didn't think I could go past that spot again. Not a pleasant experience, but it had to be done, and of course by me because no one else was home. Since then I've killed many. I don't enjoy it but it's a necessity when you have pets, kids/grandkids and lots of acreage for the snakes to hide out in.

Now just in case you or anyone else thinks that's a drastic measure, I assure you it's not. A friend who raises and keeps snakes for a research company taught me a few things about them and one is that they will often return to the same location if it provides water and shelter. If you're growing more than dirt, you'll have plenty of both. So while a spray from a water hose will often work, it may just be temporary.

My advice to you is to always be on the lookout for them and have a very sharp flat head shovel at the ready. And don't be afraid of them, they move very slowly and usually away from you. They really want nothing to do with us humans and will go in the opposite direction if they can. Most people are bitten from startling the snakes or from being idiots and messing with them.

I do have cats and they will keep the snakes away to some extent. Mine have warned me that one was near, like the one that got IN MY HOUSE! Ah, you should have seen me getting that guy out and onto the back deck, then the dirt, then making a mad dash to find the shovel, hoping Mr. Snake wouldn't scurry under something before I could behead him! Plus, I had to try and keep all three cats away for fear they would get bit. I did more than the beejeezus dance, I tell ya. ; ) My DH likes to say cussing was invented with the advent of plumbing. I say No, it was when the first cavewoman had to kill a snake! lol!

So be brave, my friend, keep an eye on your kids at all times and the other one on the lookout for snakes. And keep that shovel sharpened and ready for a beheading. : ) You can do it!

Diana