I understand this is the gut reaction of a lot of people, but the bottom line is either a young child can or cannot get out of a house in the event of a fire. If the front door is unlocked and so is the bedroom, they can potentially get out (not that most probably would make their way out in a fire...). That means that they could also potentially get out in the middle of the night when there is no fire. Which is not safe. I just saw something on the news about a four year old sleep walking on the highway. Alternatively, a parent can lock them in their room which prevents them from leaving/falling down the stairs/etc. but there is a fire risk, sure, maybe. I think a fire is less likely than a fall down the steps at night, etc, and firefighters at least would know where a child was if they were locked in their room and could rescue them. If not, they could be anywhere in the house or outside.