I'm one of those people who will call. I guess I'd rather be safe then sorry. But it depends on your definition of child abuse.
I get that people need to discipline their kids, and sometimes when the child acts up, that needs to be in public. As embarrassing as that is for both child, parent, and by standers. But when someone is DRAGGING their child by the arm relentlessly, while the child tries to pull away, is child abuse. Stop pulling and let the child sit. Then talk to them for a bit until they are ready to keep walking. You could easily dislocate an arm by pulling. You may not think your pulling hard, but if the child tugs back too hard and fast, then that's the result.
Also, people who continue to smack their kids upside the head several times, picking them up by the back collar of their shirt, to haul off and hit them again, is also abuse. I used to work in a children store and on more than one occasion I have had to call security on parents like this. And sadly, its usually the mother. Not the father. So yes I would Call for those.
But if a kid is acting out, screaming and yelling, and the mum or dad is having trouble calming them down and can discipline them without resorting to hitting then I'd have no problem. I believe in spankings. I was spanked, and most kids my age were spanked and the majority of us turned out fine. Because we got it through our thick skulls that we should never do what we did. And then never did it again. But leave the spanking at home, not in the public eye.