Child-murderer or child murderer?
Well, first of all, Archer got it backwards. To call someone a Child-murderer makes it clear that the victim was a child (though not necessarily certain that the perpetrator was an adult). To call someone a Child murderer leaves the age of both the perpetrator and victim open to interpretation (although at least one of them must have been a child.)
Compare Cop-killer , or, for that matter, baby-sitter, wife-beater or mother-f%#er