No. Even if you hadn't put a kid in the water at all, just filled the tub and left it standing two days, it still may not be fit to bathe in. Didn't you ever see those things on the TV news in the summer about kids who get sick from water left standing in wading pools? Granted, the tub is inside, so probably there are fewer ickies than outside, but there are still plenty of things that can grow and breed in standing water. I don't think I'd do it, and I am not a germphobic person AT ALL. Skin cells and germs from saliva and the odd trace of fecal bacteria and all kinds of other lovely ickies are there in used bath water, too.
I am a big fan of co-bathing. I also take my twins out of the tub and immediately put my five year old into the same water, and then DH takes a bath in the same water afterwards. But that's all in the span of a half hour. Two days, to me, is too long, unless you're going to filter and chlorinate or salt it and call it a hot tub or a swimming pool.
If water conservation is important, filling the tub only half as full each time will save more water, since you're not having to add hot to warm it up each time. Flushing only for poops will save plenty of water, although you may need to clean the toilet bowl more often. And wearing clothes several times before washing, if they're not obviously soiled or stinky, will save a lot of laundry water.
I use our bath water to water the garden sometimes (or the lawn if you have one) since our bath products are all biodegradable and non-toxic, and most of the time we only use plain water anyway.