Yes, taking a child to a movie theater is rude. I'm willing to bet, however, that virtually everyone here has been taken to a movie theater as a child. So who are you to complain, having been in that child's shoes?
Taking a child on an airplane is even ruder, but when you have a small child whose grandparents are far away, you have to occasionally do it.
Having a child is a very rude thing to do in general. They scream, they stink, and they're covered in snot. Everyone would be happier if they just weren't around to mess things up.
I find much more of a "Hey, this is my world" attitude in those who demand silence from children and hermitage from parents, than in those trying to show their kids a good time.