Why is this rated PG?

I was supposed to take my 1st graders to see this movie for the end of the year field trip. I think the parents will object because of the rating. If I could give them valid information to ease their concerns it would be good. Does anyone know? Is the fact that it's PG something silly like in "Stuart Little" because of his backside in the washing machine scene? Or is it something I need to be concerned about?