You made a decision so quit worrying about it. However, I personally think that IF you were going to do something you'd have done it and you wouldn't be on here asking for justification for your non response. You're mad at yourself for freezing up or not taking action and you want to hear that it's okay for how you reacted. It's akin to the "you're lucky my friend was standing there or I'd have kicked your ass" comments so many make.
As for the person who said the police would try to arrest you for defending an intoxicated person who put his hands on your son, I highly highly disagree. In fact, I'd venture to say that he would have gone to jail for one: being publically intoxicated and two: Assault by Contact on your son or more if I could push the issue with the DA.
HA!!! God help you than if you live where I do - Peoples Republic of Massachusetts!!
My cousin lives in Texas and she carries a 9mm Rugar on a gun belt. She could probably shoot the son of a bitch and get some kind of public service award.
If I even restrained the guy here in Mass. I would probably be convicted of aggravated assualt or something. You wouldn't beleive the shit that goes on in the courts in this state. Simply put, your logic, while certainly valid, would likely get shot full of holes somehow!