M.S.
When this happens to my daughters I tell them to tell the other person "No it's not nice to _____" and if they need to tell someone but never hurt back. Our school teaches "use your words" and "gentle hands". And don't hesitate to gently but firmly reprimand the other child in a way that hopefully won't set the other parent off. I have done this before to... interviened in the situation and told the other child No No that's not nice we can't hit etc. It's hard to see that happen to your child but that fact is it's gonna happen and we just have to arm them with the words to tell the other person, the wits to walk away and the smarts to know when they need to get some help for the situation.