Regarding the idea that having a small lock on the outside of a door is somehow dangerous if there is a fire...
I believe most young children, if frightened in this sort of situation, tend to "hide" under a bed or in a cupboard etc. A firefighter friend of mine has experience of this sad fact.
So, having a small, easily mashed lock will make no difference to a rescue operation if there are responsible adults around. The same reasoning is applied to babies in cots. You wouldn't EXPECT a young child to evacuate THEMSELVES would you? I believe it is only the same as a stairgate over a door and no-one bats an eyelid at that.
We used a small, flimsy bolt on DS's door when we first moved him out of a cot (as a last resort). He WOULD NOT STAY in his bed. He would wander around at night and we have very steep stairs. I preferred this to finding my child with a bashed-in head.
After a few weeks, we found that just the THREAT of the lock was enough and the need for it was no longer there.