Assuming living in the civilized world, I'd call the police 'cause I'm not physically capable of stopping the callous parent.

P.S: In my childhood I've seen lots of parents beating their kid in the street. It was kind of a norm in this shithole. I myself even got beaten by my father in the street.

I'd think he was an asshole child abuser, I'd feel bad for the kid and the lifelong psychological damage he'd probably have, and I'd call the police but he may leave before the police get there. I'd want to attack him but I'm not a big guy and I've never been in a fight.. I'm not sure whether I'd do something physical or not. Oh, one good idea I might have: capture on video with my cell phone to show the cops.

Time to call the popo.

not my business 😜