personally, i would beat them to death from the feet up. baseball bat, cigarette burns, and some kind of public humiliation would probably be warranted as well. i would prefer a method of death that allowed them to believe they were being watched by their victim and also allow them just enough concentration to be reminded of what they did to her for years while they themselves suffer. children are by nature more innocent the younger they are.
problem is, no amount of punishment can put someone into remission of sociopathy. there is no hope for them so death is simply necessary under all circumstances. they cannot be 'taught the error of their ways' by pain or anything else. they just need to be gone in such a way as to give anyone they hurt as much closure and satisfaction as possible.