Em, we've all 'seen parents smack their children so hard that they were left in flood of tears', most of us were probably smacked so hard we were left in floods of tears, not really through the actual pain though. When the parents are smacking kids and leaving marks, then we have a problem, but when they're smacking an unruly child on the bum or the hand, would most people bat an eyelid!? But yeah, that's 'technically' assault!
We could all through extreme comparisons out there, it would be assault to 'man handle' a drunk man from a kids playground who was screaming obcenities, you'd still hope someone done it!