You'd think that would have worked. When I was a kid I remember being completely mortified if some strange adult told me off. You get orders and such from your mother all the time, but if you do something someone else expresses disapproval of, you straighten up right quick. Or at least that's how I remember it.
But this kid... As soon as I had finished my question he looked at me in indignation and said to me "My mom can tell me that."
Luckily his mom was nearby to help out, because in this kid's eyes I had absolutely no authority.