My father in law slapped my son.

Hi Ladies, I'm posting here as I am upset about what happened this afternoon with my father in law. They have come to visit us (him, my mother in law and their 2 teens) and staying in their own appartment. Today my son was hyperactive, just turned 4 years old a couple of days ago and well..he likes to press buttons put it that way. I'm all for giving a child a tap on the bum, but today my son had pushed the boat on my father in law and his last antic was hitting the TV over and over and for some reason my father in law grabbed him and slapped my son in the face.

I didn't realise what had really gone on and how hard my son might of been slapped..there were no red marks but two whitish lines on his face slightly visible. Automatically my father in law was sorry, I could see he was upset with himself..I didn't shout at him or do the whole this is my son thing..I think I was a little shocked myself? He said he was sorry over and over and I told him not to worry...i feel like a bad parent in away for not being angry with him..as soon as they left I crawled into his bed to hug my son..he got over it pretty quick but basically my question is

am I over reacting..do people make these mistakes? I don't know..thanks for reading