I don't quite follow. If a child dies due to abuse by a single parent, who is it who you suppose would become a vigilante? The child is unlikely to have had any adult friends or relations that would want retribution so much that they kill the late child's parent.
If this was manslaughter/murder of an unrelated adult, then I could see that there could conceivably be some friends or relations inspired to vigilantism, but this thread has mainly been about manslaughter of children by parents due to incompetence.