What Do You Do When You See Someone Mistreating a Child?

Last Saturday I was at a wholesale club. As I stood there trying to decide if my family could polish off 5 pounds of chicken nuggets, I suddenly saw something shocking: A fellow mom, irritated at her son, hit him HARD on the back of his head with her cellphone. The boy stumbled forward. She jerked his arm. 

"Hey!" I said sharply. The woman glanced my way, obviously startled that someone had been watching. She began slinking off. And then it was on....

"HEY!" I repeated, so loudly dozens of people halted in their tracks. The woman gripped her son's shoulder, steering him away. I followed. "YOU DON'T HIT A KID ON THE HEAD WITH A CELLPHONE!"...I was screaming now. "WHAT KIND OF MOTHER DOES THAT?" Without looking back, Mean Mom muttered, "He wasn't behaving."  I screamed, "I DON'T CARE!!!". She raced for the exit. "Go quick because I might call the police!" I called after her. 

I'm still thinking about it. I couldn't help how I felt, yet in the heat of the moment, I disregarded all the advice I've ever heard about what to do when you see stuff like that go down. You're supposed to sidle up and say something like, "I know, it can get stressful at times, can I help you?" (I suspect she'd have told me to mind my own biz, and not in that language, either.) Should I have called the police? She'd have been long gone by the time they arrived. Did I make it worse for that kid ("Just WAIT till we get home...") or even put myself in danger? Her husband or boyfriend could have been elsewhere in the store, and she might have gone crying to him. Should I have told a cashier - what would they have done? 

And if you were me...what would YOU have done?