Sorry, I disagree. I think it is disrespectful for kids to sit down when there are not enough seats to accommodate kids and adults.
seeallsides:- Until next time you get into a full car in the NYC subway system. People don’t stand up too quickly for anyone on a full train, but the moment a child walks on, there are always several people who will jump out of their seat to offer the child their seat, even if the parents have to stand up, and even if the child spends a substantial part of the ride, standing up and not utilizing his/her seat, but G-d forbid that anyone should even contemplate sitting on that seat & perhaps depriving this child of his/her seat if/when this kid finally decides to sit down.