Definitely got to put some pressure on the kids to do the right thing. It's called parental guidance. Can't let them go off supporting whoever they want, just like you can't let them eat what they want all the time - otherwise you'll end up with obese kids in ManUtd shirts everywhere!
Exactly, class mates at school can brainwash others, my son had a birthday party when he was 11 and most of his classmates turned up in Arsenal shirts, and he went to school near Lewes, I was very proud that he didn't follow the crowd and that he decided to support my team.