She was a perfect child

I know I know, there is no such thing as a perfect child but I like to think that Savie was a really good child. She listened really well, talked a lot, loved to play with anyone, willing to talk to just about anyone but things have changed. And not for the better.

Since we have been going to preschool she has become a sassy little thing. She tells me 'no' a lot more, has temper tantrums and has just picked up some really bad habits. There is one boy there who is the picture of an only child. He is used to getting what he wants when he wants it and when that doesn't happen he throws a fit. Now myself personally I am going to ignore that kind of behavior. My theory is the more attention you pay to it the bigger it gets (you know the squeaky wheel and all that). He wants the chair that someone is already sitting in, on the swing that someone is already on,etc and when he doesn't get it right that second he huffs, crosses his arms and then throws himself on the floor crying. Savie has started doing this and I am not having that. It's one thing for the parent to treat their kid that way but when they go to school they can't do that.
Am I wrong for thinking this? I can't tell the parent this because I think I already offended one parent by telling them to drop their kid off and go instead of dragging it out for a good 20 minutes and the kid ends up crying. I just don't know how to say it without offended them. But there is no way to say that without offending them, right?