Bathtub/shower combination with the shower on a handheld unit with a slide bar. The handheld unit is great for washing little ones in the tub and as they get older they can begin to shower with the head lower on the bar.

Shower/tub and lavatory faucets are easier to operate with lever handles than knobs for young hands (and older ones, too!)

I liked having the shower doors with DS - once in a while we would just close the doors and let him splash to his heart's content with no worries about water on the floor!

Extra floor space is really nice to have. Imagine kneeling on the floor drying off squirmy toddlers.

Storage is always good. Hooks are easier for younger kids to hang towels on than bars. Teenagers don't hang up towels, no matter what is provided.

Having said all this, they are little for far too short a period of time. Don't feel the need to make drastic changes to accommodate young kids that you won't find useful later.

Space between the toilet and tub front is more important than anything. Standard code spacing is only 15" from center of toilet to the next object (tub, or vanity); so 30" for toilet space. But, if you can make that 36" or more, for extra tub clearance, that will help you the most so you can kneel at the tub and have full access without having to reach/lean over a toilet bowl. It also makes turning on the water/adjusting the temperature of the tub water easier.

The lower height toilets are good in rooms that kids use. In general, if you can afford it, I think a trough sink or double vanity pays off in a space where 2 or more children share morning hygiene functions. Jack and Jill was our choice, where each child had their own private vanity and shared the toilet/tub area.

You may want vanities that you can tuck a step stool under for right now, but are conventional height otherwise, because the kids will be bigger much longer than they are smaller.

My house growing up has no medicine cabinets. We had drawers for our stuff and there was a cabinet with a three tiered lazy susan above a bank of drawers. Stuff for kids to access on the bottom tier, stuff for adults to access on the top tier. All of which could be spun around to the front. There was no "back of the shelf" in that bathroom. We were old enough when we moved into that house that heavier security than that was not needed. (Like a locked cabinet)

Having the toilet next to the tub is great when you have toddlers-elementary age kids who need to be supervised, but don't need a lot of hands on. This is where Grandma sits to supervise the fun.

Having bath towels and washcloths at hand is important. My daughter, for lack of other space, installed one of those over-toilet wall shelving units. Hers doesn't have doors. She rolls the little kid towels, etc. and puts them in baskets which sit on the shelf. I like not having to open a door to get a washcloth.

Thank you all so much. This is hugely helpful. I never thought about the hand held unit with slide bar, but it makes so much sense to me now!

What about bath gear and toys? Aside from foot stool to the sink, what other stuff should I expect to have in the bathroom? How much bath toy storage is needed? Should I bother building extra ledge by the tub to set toys on or is there better solution than to waste space with a ledge?

Don't bother with a ledge. The toy stage doesn't last that long, and a ledge is hard to drain right anyway. Just get a basket, or bag, or choose only a few toys, and it will be fine.

Other gear was a step stool for sink usage (or a toe-kick step); cups (for rinsing); and toilet training stuff. All short-lived and no reason to specially plan for, imo.

Also, I prefer having a single-levered faucet at the sink rather than something with 2 handles. The lever makes it easier for them to turn on and off too, instead of a handle. Single is better than double because more often than not, they will turn on only HOT water.

Then, for safety, of course you need to make sure your water heater is adjusted to 120* maximum so when they do turn on hot only, it isn't as scalding.

Thanks everybody. After taking everyone's input, my architect drawn up this floor plan. The countertop is not as long as what i would have liked, but the space in front of the tub should be really helpful with bathing the little ones. Excited!