I think there has to be a distinction between an ideal and the reality. If it were demonstrated (if) that there were a certain number of select situaitions where negative reinforcement of a bad behaviour by a light tap on the bum would lead to a good outcome (better behaviour), this would be a good thing if (the ideal), the world were populated by knowledgeable, self-controlled, disciplined adults. It's (reality) not. It's full of people with poor self-control, very little knowledge about the actual effect of their actions, who are normally just acting out of anger.
THEREFOR, it is by FAR the best course of action simply to place a blanket ban on any parent>child assault (I mean any form of physical action to a child that among adults would be classified as assault). Normally, the effects of the type of behaviour seen by adults to children is very negative and seriously damages the child's mental wellbeing. Just because the parent can't control themselves and justifies this lack of control as 'teaching them a lesson'.
It should be easier for people to report parents who are abusive to their kids. It goes on a lot.