Parent's aren't allowed to abuse their children, the law is not different between strangers and family members. I hope those people will be jailed for the rest of their lives. Really, death is too good of a faith for them.
However, as far as your question goes. there is a law keeping parents from abusing their children, if anyone would have reported or complai, that child's live might have been saved. I think it's vary hard to tell if someone's being an abusive parent, especially when the children ar too young to talk or understand what's going on.
You know that phrase "it takes a village to raise a child"? that's one of the reasons - so that there'll be more then the parents responsible to the life and happiness of the child.