I'd start over.
Try a row cover or caging to protect them.

ychays, little trick, very easy (and free) that might help: cut toilet paper rolls in half or thirds and place around seedlings. This will work for some things only, but it is easy enough that it is worth a try. I live in your area and it could be so many things, including squirrels, birds, slugs, possums? I lose a lot of things this way too.

I think your culprit is a rabbit. You can fence them out with rabbit fencing or 1" mesh chicken wire. Bury the bottom edge of the wire or fold it outward so that about a foot of wire is flat on the ground facing away from the garden to keep the bunny from sneaking in under it.

If the plants have been nibbled right down to the ground, best to start over. Even if they recover, they'll never catch up to a new planting.

Honestly, you'll be wasting much of your time in starting over if you don't first ID the culprit or culprits and control for them. They will just go after the new plantings too.

So take the time to explore and watch the plants at various times of the day and night to see what it is doing the damage. This means you have to check the garden several times a day and again in the evening. Your comment "growth I saw the other day" sounds as if the plants aren't getting monitored enough so new plants will just disappear too.

Many times it is a caterpillar that is easily found if you look closely, sometimes rabbits, squirrels, groundhogs, birds, deer, etc. and there are many discussions here on how to control for all of them. But first you have to find out what it is and unfortunately, only you can do that. Sorry, but pest prevention is all a part of gardening.

Dave

Thanks, everyone. I've started to check almost every couple of hours. This morning something small and black took off and dove for the bushes between me and my neighbor's yard. But, seriously, it went so fast, I couldn't ID it. A bird, maybe a small bunny. I see a lot of sow bugs in my yard - could they do that kind of damage overnight? The reason why I think it's mice is because I saw some dropping on the pak choi that resembled mice droppings and I remember in the house inspection there was some note of it. I am going to try mice traps around the vegetables, a cover for the new seedlings along with the toilet paper rolls. I've raised the seedlings from seed so it's so hard for me to see them go...

In Glendale, I'd suspect birds and snails.

Then, if you're in the hills, the larger varmints are other possibilities.

If you have access to fine wood ash, try spreading it around the affected seedlings. It will discourage snails and you'll be able to see small critter prints in the ash and possibly i.d. from them.

Great idea to use wood ash to make footprints visible. Flour works, also. If the damage is being done by a critter and not a bug, try hanging repellent balls from short sticks along your newly planted rows. A repellent ball is just the toe of pantyhose (or a cheesecloth bag) stuffed with dog fur(any fuzzy dog will do, except poodles since they have hair not fur). The scent of the dog fur repels marauders. The fur should be replaced every few weeks, depending on how much rain you get. If you don't have a dog and don't want to get one to have a handy supply of fur on hand, try any grooming facility--they usually have tons of it!

You mentioned sow bugs, and I too have them all over my seedlings. I've heard it said a million times that they are only interested in decaying matter, but I sure think otherwise when I go out to my garden at night with a flashlight and see 30 to 40 sowbugs eating my broccoli seedlings down to little nubs. They did the same things to my peas, my arugula, my kale. Take a flashlight out there at night and see for yourself. I'm in San Pedro, a bit south from you.

I wish I had some photos from a few years ago. Sow bugs ate half of a cantaloupe. Yes half. I've also seen them munch on an overgrown zucchini. I've had them eat a whole row of sprouting green beans and chard. Impossible to grow strawberries. Maybe in a container.

I'm sure in other areas it is not a problem. Actually I don't remember them being a problem here when I was young but they sure can be.

So yes, elbeardo, I believe you!