I don't see how there can possibly be a happy ending. Even if Madelaine turned up now safe and well, the family could hardly go back to their previous lives as though nothing had happened.
I can't make up my mind about this case except that if the parents did kill her, I'm convinced it would have been accidental.
I also read that using the child-minding service would not have made much difference as they only listen for a crying child and if she was removed from the room silently they would not have known about it.