such a difficult situation this is. i know that as grown-ups, it is definitely our responsibility to make sure that all kids are safe and fine. however, it can really get difficult to understand when we really should get in the picture and when we should let it be.
sometimes, yelling is still fine. however, if the parent was outright physically abusive, i would say it is important someone interferes. i once did, and even though no one else supported me, and i looked like someone who was getting into another's business, i just couldn't let the mother hit her child the way she was doing. my interfering did make her stop and go away, though i don't really know what happened afterwards.