Never. Physical violence committed against children is an abomination. There are other, more effective ways to introduce discipline and promote safety. Violence is the tool of the oppressor. There is no love behind it, only fear. Often, I suspect, fear of appearing to be a bad parent in the eyes of those around at the time. Even if it is "well intentioned", it is wrong, and does only harm, causing pain, humiliation and promoting violence as acceptable means to attain an end, which sets the child up to repeat this pattern in their own interactions in life. There are only a handful of things I have a very strong opinion about, and this is one of them.