You should have got the dog then informed the mom. You never put your hands on a child especially if they aren't yours. He is five years old and you say he should have known better? I have raised 3 sons and 5 year olds don't have the full understanding of their consequences yet. He should have for sure been punished and corrected , but not by you. If anyone put their hands on my children they would have a big problem , big problem. I ,as a mother, would have my eyes on my child the whole time and for sure would have made him/her apologize and I would be so very apologetic, but you would have had a problem once you put your hands on my son. I don't mean to be rude, but it's just how I feel.
