Why not well designed large Children's Bathrooms in large family homes? For toddler's bath, pottie. Not Guest?

Why are large ';family'; homes designed with ';guest'; bedroom %26amp; bathroom and not for babies, toddlers, Children and teens?Why not well designed large Children's Bathrooms in large family homes? For toddler's bath, pottie. Not Guest?
A child can use regular sized bathroom facilities. For example, when I was a kid, we had step stools to allow us to access the sink to brush our teeth and stuff. But, an adult cannot easily use miniature bathroom facilities and it would detract from the value of the house (you would have to remodel to standard sizes when the kids grow up). Although, I have seen miniature toilets from American Standard that you can get for children and little people. Interestingly enough, they actually cost more than their full size counterparts.