It's this incident that makes me quicker to act in situations like yours. I imagine you will never forget this incident, but it possibly will change the way you react in a situation in the future. And btw, I would have been worried that he'd follow me to my car too...so I would have stayed in the library until he left! I am more inclined to stand up for people, but I'm still a big scaredy cat
ETA - given this was at the library, maybe you'll run in to this man and his daughter again and you can say something really positive to the little girl and build her self esteem up a bit (not about Santa...just pick something that relates to what she is doing that day). It doesn't matter what you say, if she spends her life being belittled by her father, and watches her mother being belittled, she'll probably remember those words forever. I don't even know what the girl or the mother looked like who was whacking her daughter because I thought nothing of a mum and daughter going to the toilet until I heard her hitting her behind the locked door so I will never be able to help her. It's been years, and I can still hear the whacks as clear as anything when I think about it.