A good idea. Like Esspee I thought it was already in law in the UK. Or something nearly there.
My only concern is when is a child a child. If a child regularly sees a parent hitting or beating the other parent, s/he may well think s/he has the same right, especially if s/he is in a gang where violence is the norm. What right would the parent have to defend themselves? A thirteen year old can pack quite a wallop.