It bothers me too when I see peoples kids without a yarmulka but instead of screaming at the parents you can nicely tell the kid to put on his yarmulka.
You should know when I see Jewish kids fighting or doing something wrong, I often go over and tell them in a very friendly non threatening tone of voice that “Jewish kids or (Yiddishe kiderlach if they are chassidish) dont act this way” they usually get the message.