Each case is unique, there are no generalisations possible.
Fudge. The fact is - you have absolutely no proof that Madeleine would not have come to harm even if she'd been locked into the apartment with both parents in the bedroom next door. The McCanns did not
allow her to be abducted, nor IMO did they deliberately put her in harm's way as some people are keen to insist. If Madeleine was abducted (which it seems is by far the most likely explanation IMO) then the ONLY person to blame is the one who took her, this is backed up by the legal system which has never sought to prosecute the McCanns for neglect, nor as far as I'm aware has any child's parents anywhere ever been prosecuted because they "allowed" their child to be abducted, either for allowing them to play out past bedtime in the dark, or by losing sight of them in a shopping centre, or by leaving them alone in the bath while they went upstairs to fetch something.