I had something similar happen when I was with my daughter at the playground. Some little boy was acting a bit aggressive toward my daughter, but nothing I was to worried about. Then I saw the dad on the other side of the playground who was watching what his son was doing and not saying anything. The dad couldn't see me and then I stepped around the playground and when he did see me he instantly shouted out to his son - "son, make sure you play nice!" I just smirked and stood there.