I think it's all a bit suss... if it was a weirdo, they would have surely found some sort of trace, as they didn't have much time to do a proper pro job of it and like others have said why would they not take the other children too? So I would assume the parents either knew about it in advance and let someone else go in there, or they themselves did something. They seem a bit odd to be honest.
I too think it a little odd they haven't been queried as suspects before.
Woo ok, time to run for cover now...