I don't think the door being unlocked means they usually keep kids locked inside. I think the door being unlocked means someone left through that door and did not lock the door behind them.
My kids are 4 and 5 and are not kept inside by a lock on a door. This does not strike me as the type of family that locks doors to keep children inside. They seem to spend a ton of time outside. I think they lock doors to keep people outside, like most of us do, and the door being unlocked means someone left thru the door and did not lock it behind them from the outside.