The problem is that to some extent, parents have ownership over their children. To what exact extent this is/should be the case is of particular interest to libertarian circles (i.e. how does the non-aggression principle apply to parent-child interactions?). Obviously it is wrong for a parent to abuse a child, but its hard to draw a line between acts which are abuse by almost anyone's standards (i.e. murder, severe injury, rape) and those which leave room for debate (i.e. indoctrination or other mental abuse). I would tend to be conservative and stick to prosecuting the acts that provably cause irreparable harm to children.