I understand why they might choose not to have the small toilets; they would just be convenient with small kids.
I have seen the flip down stools in lots of public restrooms just not at Disney. They are completely out of the way until you need them, then just flip down under the sink for the child to stand on. It's much easier than trying to hoist a kid up to the sink while juggling a bag and trying to keep clothes dry and then trying to turn on the water. Luckily DD is tall enough to wash her hands by herself now, but there were so many times I wished for those at WDW.