I think the person they really need to be questioning is the friend who claims that he went to check on the children but only listened at the door. Why wouldn't he go in? If I was going to check on a small child and heard total silence, I would be even more likely to go in and make sure that the child was, in fact, still in the room. It's very fishy.
I think that he was either directly involved or just knows a lot more than he's letting on.