All kids could be in danger at ANY time. It was dangerous to do what the child did but its not like the parents intentionally let him out. He even had to unlock his way out of his own house.
Anyways if you ever have kids tell me. I'll keep a 24/7 eye out for them and if they ever do anything that could hurt them I'll call CPS on you and you can have your kids taken away from you and have people calling you a bad parent when they don't even know the full story just like you are here.
Then you can tell us exactly how it feels to be held accountable for something you don't always have control over.
I think that CPS should look into it, and then assuming there are no other incidents rule that "kids will be kids." On the other hand if this is the 10th time the kid was out wandering around, something is wrong.