My mother was a 'public displays of violence' type... so I was that kid that folk thought 'should I say something'...
If a parent is abusive in their handling/care of their child, you saying something isn't going to improve things unless:
1/You can report something tangible to the police or SS. A location, a car reg, an address.
2/They are actually about to kill the child and you can stop them.
For most people in most instances, neither of the above are true and I can tell you, as the child of that parent, when someone approaches your Mum, mid 'pants down bum smacks' in Asda and says 'is that really necessary'... its TEN x worse when you get home. It's all 'you showed me up in public', smacked arse, smacked face and bed with no dinner, at best.
There were days she'd appear fine in public, but we'd have mucked about (nothing truly awful, we didn't dare! It could be the wrong look or tone of voice)... she'd not say a word. But as soon as we got in the car she'd change, like someone flipping a switch, because we'd drawn attention to ourselves and thus her, but she'd felt she couldn't do anything in public lest someone say something. So then she'd be vile all the way home, ramping it up until we got in the door. Then the first one to open their mouth (usually me) would get a backhander across the face, no matter what it was we were saying!
Abusive parents either do not know how to not be abusive - in which case you saying something does nothing to stop them.
Or, they do not WANT to stop being abusive... in which case you saying something does nothing to stop them.
So if you know where they live or their car reg or you've got video evidence you can hand in... report it (please do, there were SO many people who knew something and could have made a meaningful report and didn't and at 44 I am still angry that they didn't!)... but otherwise, don't.