I don’t know, Disney literally will not let you in the line with a child that is too small. I get that it’s for safety, but you either split up or don’t ride at all. The child didn’t choose to be too small, so it’s not really a choice. Even a family with a disability could still - in theory, depending on the line and severity of the disability - choose to wait in the standby together.
(I can see it being a choice for a kid who just doesn’t want to ride but is otherwise tall enough, but I think in that situation most of the time the family sticks together and then the kid just waits 90 seconds at the exit by himself. That was always my experience anyway.)
Fwiw, I can see your point too, but I’m just trying to imagine what can be considered “reasonable.” I know this is a grey area and I’m not at all trying to argue with you.