Sorry -- I disagree.My latest mother-defending-the-cubs blow-up (about two months ago) was over even less of an issue, but regarding the same general principle.
Someone complained directly to my son about his behavior in a public place. The person who was correcting my son saw me standing right there at the time, meaning either 1) I didn't see that my son doing something wrong, or 2) I saw it but had no problem with his actions.
I took the person aside to address how he handled the issue, and that with me right there, he should have addressed me and let me deal with my son. Personally, if I *had not* been present, and some other adult had seen my son misbehaving, I would have been grateful for someone standing up to tell him (calmly) to cut out whatever, and bring me word afterwards. However, in this case, the person went straight to addressing why my 10-yr-old autistic son (who plays with his hair so much he has worn a bald spot into his scalp) should not be wearing a cap inside. Grrr!
Correcting other people's children is an emotional landmine looking for a place to blow up. There is almost no *right* way to do it, that will guarantee no one will ever take offense. Too many people see their children's behavior as a direct reflection on themselves, so you are actually criticising them, rather than being viewed as helping the child. In general, I would suggest getting the parents involved whenever possible. Then, if you see the same response, at least you will know where the kid got the behavior issues.
Maybe it's because I've dealt with too many kids whose parents have never bothered to correct them or who apparently don't realize that the actions of their little monster aren't as cute to me as they are to them, and even are sometimes downright dangerous.
But I figure if the parent hasn't corrected the kid's behavior after a couple of minutes (during which I may well have been trying to get the parent's attention)... I'll do it. With the necessary tone and mannerism. Sometimes that means simply saying "Hey, stop." Other times... well, there's something we call "command presence." It's putting that snap into the tone of voice that says on a subliminal level "if you don't comply, I'll make you comply." Yeah, I've made a few kids cry.
But, see, I don't care if YOU as the parent don't have a problem with the behavior. If it's disturbing me (recall, I try to be reasonable; the standard is different at a matinee showing of a Disney cartoon compared to a 10 PM showing of a rated R action flick or at Mickey Ds compared to a $25 an entree restaurant), it's a problem. If you don't take action to bring the kid in line, I will. Now, I'm not suggesting I have the right to do more than tell your kid to stop it under most circumstances -- but I most certainly do have the right to address the kid directly.