I usually talk to the iffending child and tell them "No. Do not hit/kick/punch/whatever" or even "leave (my child's name) alone." If they continue, I will go over again and say "No." or "don't you dare." and if they still look like they are going to do something to my child, I will find the parent and tell them exactly what their child is doing and that they have continued doing it even after being told not too. It usually doesn't get that far though.

And before then I will try and let my kids work it out or tell them to stay away from the problem kid.