I cannot get my head around that either. Imagining a scenario which fits just plain eludes me.
That bothered me too. But then I just read that the parents weren’t considered suspects until all other paths of investigations dried up and they backtracked and began looking at the parents. If that is indeed true, then they probably would not have been watched. It’s still strange, though, I agree. Everyone would have known what they looked like, so they took a big risk.