At 15 months you can start teaching her that she cannot go out the door without first asking a parent. That said, she is only 15 months and you cannot completely rely on her ability to manage her own behaviour.
Some suggestions:
- Small gate before the door would make sure she couldn't reach the door while allowing your family to step over it.
- Put a brightly coloured sign beside the bolt that says "LOCK THE BOLT FOR (insert child's name)". That way your family will have a visual reminder to lock the door and shouldn't forget. Also, tell your other children how very important it is to lock the door.
- You can purchase small plastic pegs and latches that could be used to keep the door shut.
- If this is becoming a serious issue, you can also purchase small alarms that go off when the door is opened. It simply attaches to the door and door frame.
Finally, take the mystery out of the door. She is at an age where she wants to explore everything. Don't go overboard on the door. Let her explore it, see what it is, where it goes. In otherwords, let it become boring for her. Afterwards, simply redirect to another activity rather than drawing too much attention to the door.