I have always done this, in waiting rooms, on buses, anywhere really. Even till they were too big really to be sitting on laps. Even now I will get them to sit on the floor at friends houses so that adults get the chairs. It's polite and the way I was brought up.
I often see children taking up a space, when it would be easy just to pop them on your knee for a short period of time. I know for a fact that some of these parents have no physical reason not to do this. They just think that their child has as much right as an adult to have that seat. True but it's not good manners is it?
AIBU?