There is a big difference between a smack and being beaten, do you even know what she was being smacked for? Maybe she was trying to run out onto the road and gave her mother a fright so she decided to scare her from ever doing it again.
As for shouting, well I'm glad my neighbours can't hear mine, some people are just fierier than others, obviously it's different in that she's yelling at the child, she's probably under a lot of stress and not coping very well. Those are just two snapshots of her at her worst and really they're not any worse than most, calling ss will just cause more problems for the family.
There's something very wrong with a society where people are calling ss to invade a family's life because they display less than perfect behaviour, even worse when ss encourages that sort of nonsense. There are so many children being genuinely abused; raped, beaten, starved, socially isolated etc... it's no wonder ss rarely get around to rescuing, they're to busy 'investigating' smacking and shouting!
If you want to help this woman, you could try befriending her, if you don't, that's fine too, just don't go making things worse, sounds like she's got enough stress in her life.