the kids shirt. Now, maybe it isn't common sense to have a U of U football player sign a Y shirt or even to wear a Y shirt to a U of U camp but still, you don't ruin a kids Y shirt when giving an autograph.
Having said that, I am picturing the kid wearing a BYU t-shirt. Maybe he was wearing a shirt that said Be Happy and Harvey crossed out the Y on Happy or some similar situation to that. If that is the case, no problem at all. 2 rules of general etiquette were broken if it played out as I have in my mind from the info given. 1) You don't wear a BYU shirt to a U of U camp. 2) You don't cross out the Y on a kids BYU shirt when giving an autograph.