My 10 year old is a Fin fan, my 8 year old is a tweener right now, he likes to play football (real or playstation) but he dosen't have the attention span to sit and watch a game yet. My wife thinks he'll be the one to test my patients and go with another team, but from experiance I know how to deal with this, just let him cheer for another team, because if they think it bugs you they'll keep on being a fan of the other team just to push your buttons.
My nephew did this to me, I told I didn't mind he was a Pats fan as lomg as he enjoyed the game of football, of course I was seething inside with anger, but eventually he came around and is a huge Dolfan.
P.S. This also works if your daughter is dating a kid you dislike. Trust me if they think it bothers you they'll date him just in spite.