I'm kind of expecting a range of opinion here (including some who say yes just based on the title) so here's the story;
I went to the pub today for lunch, local village pub, fairly family friendly with a play ground (slide etc.). We'd had lunch and my wife and I were sitting at a table just next to playground whilst DD (3yo) played on the playground. I was keeping half an eye on her and she was walking up a ramp (to a bridge and eventually the slide) when a girl who was sitting at the top of the ramp (looked about the same age as DD) kicked/stamped on my daughters shin (seemily unprovoced, although I couldn't hear if DD said anything but she's fairly shy so I doubt it). It wasn't that hard, and DD just tried to step past (it was a fairly wide walkway) so I didn't do anything but kept watching. The other girl reacted to DD steping past her by punching DD twice in the chest then once in the face. DD covers here face up obviously crying so I ran over to check she was OK. I said to the other girl "No, we don't hit. That was naughty." or something very similar, admittedly in a fairly stern telling off voice but nothing more, and carried DD back to our table to check she was OK and calm her down. About 30 secs after I got back to our table I noticed the other girl was clearly a little taken back and shocked (although not crying) and was considering whether to go and try to find her parents with her (I'd assumed they still hadn't seen anything that was goin on). At that point her father came along and asked what had happened, so I told him she'd punch DD and went back to my own family. About 10-15 minutes later the girls father came up and started giving me grief for "getting in his girls face" etc. We had a bit of a fight in which it became clear he hadn't seen any of the incident and was just accepting what his girl said (for example when I pointed out his girl has punched my DD in the face he said DD had pinched her to provoke it - I know this hadn't happened as I saw the whole thing).
That's about it, ask questions if there's anything I've missed, to what extent AIBU?