It wasn't 40, it was 10, up until that point Joshua had been popping back into the bedroom every few minutes. That's why when 10 minutes passed with no sign of him, it was enough to alert her that something was wrong. That is the story that we keep hearing from mom and apparently, that is supported by dad. I can't tell you how many times I've had my kids in bed for an hour, and all three adults here will suddenly realize that we are watching Handy manny or Mickey Mouse. And she may not have been just sitting there watching the movie with him. She may have been trying to relax or get him to lay down with her. Remember, she is almost 9 months pregnant. There are times at that stage of pregnancy that you just don't want anyone else around. (Kids are different.) And remember, she knew he was mobile, but so did his father and the five other adults that were just a few feet away.
Someone said there is footage of the dad going outside to have a cigarette. Did they have a strict no smoking rule in the house? To where more than one person could have been ducking outside multiple times to smoke? That wouldn't raise an alarm, might not even raise a memory. And it would be easy for a door to accidentally be left unlocked and/or open if you have a lot of people ducking in and out for a mundane reason like having a cigarette.
I just don't think the parents did it. (Of course, I really wanted to believe that Caylee had a nanny too, so it's not like I've never been wrong.)