The only reason why I don't think this can be the case is, SW would have checked on the girls, as they slept. I know when I come home and my kids are asleep, I have never, not ever, not once, gone right to bed and not checked them, as in went into their rooms, looked at them asleep and THEN gone to bed..
That said, maybe not every one does that..it was late, she was tired and she was pregnant. But, I just still can't see her not looking in their rooms and they were gone or deceased then CW strangled her as she was confused from the sight of no girls in bed, or deceased girls in bed..(sorry for the TMI) its all so grim and unfathomable..