If you feel so strongly, perhaps you should say something to the woman and warn that if you see her doing it again, you will report her to the police and social services, never mind the NSPCC.
We all, (including me!) try to avoid difficult situations, but if it was as horrifying as you say, is that not the time to stand up and be counted? The problem is, I suppose, that you don't know if it will just drive the behaviour underground, so she does it where nobody can see and the bruises don't show.
Once many years ago, we lived in an upstairs maisonette and going out, heard a commotion in the downstairs maisonette and looked in to see our fiery downstairs neighbour laying into her 5 year old boy with a Prestige Fish Slice. We didnt need to say anything as she stopped as soon as she realised we had seen what she was doing, but I often wondered if we should have reported it, and if she ever took it out on him later when nobody knew.....