Sorry that sentence was REALLY badly written for some reason, so I am going to rewrite it.
@ Undertaker, if someone even contemplates killing their own daughter they must be at least disturbed, if not psychopathic. A psychopath in particular, could sit there (not their XD) very calmly. Remember - they went to check on their children every ten minutes, who is to say something didn't happen then? It could of been only one of the parents.