Since this made quite a splash in the news, someone did look into it.
Turns out, a very harsh law against domestic abuse was passed in 2016, but it had a number of problems, one of which was that physical violence against a relative led to harsher penalties than the same kind of violence committed by a stranger.
TL;DR, if you wanted to discipline your kid, it was better to let a stranger do the smacking - no mandatory jailtime for anyone that way.
