And to be clear, you are allowing assaults on those who would have the most difficulty using the legal system as a post assault recourse, and who are in relationships where there is an imbalance of power to start with.
Yes of course. I think it should be difficult for a child to use the legal system for recourse after getting slapped / assaulted unless it was beyond reasonable chastisement.
The imbalance of power is essential in a parent-child relationship IMO, provided it's not the child who wields more power than the parent, and provided the parent does not abuse their power by going beyond reasonable chastisement.
Children can't handle power - usually their brains are too under-developed and they don't have life experience compared to most adults. Someone in the parent-child relationship has to make decisions, and I would say that someone is the person who could be held legally accountable for decisions made - so the adult. Of course there are exceptions where I feel the law should step in but not for a slap IMO.