Prevention that didn't prevent

A couple weeks ago I noticed that my bedroom door has a doorknob meant for an outside door (so it has a key hole instead of a round hole to push a nail through). I noticed this because Kaleb liked to go in my room and shut and lock the door. At that point I would tell him to unlock the door and in the midst of giggles he would.
I realized that it was possible that sometime he could lock the door and then something would happen so he couldn't unlock it (such as getting hurt). So, after trying all the keys I had in the lock I took the handle out of the door, to a locksmith. He re-keyed it and I put it back in. So the next time he locked the door I got out my apartment key and unlocked it.
Fast forward to this evening. Kaleb was being crazy and running around the house. I heard a door slam and little feet running. Kaleb had locked his bedroom door, and then shut it, from the outside. No problem right? Wrong. His bedroom door also has a keyed lock instead of the kind that has a hole for a nail to unlock it.
I called my landlord and told her my predicament and asked her if she knew a locksmith. She said to go ahead and call one. When I asked if I could just take the cost out of my rent she hesitated and said she didn't think she was responsible for that since it was my son who locked the door. I responded that although that was true I should have been given all the keys to the apartment when I moved in, (thanks Lydia).
To make the rest of the conversation short: her son is coming over tomorrow with a drill and a new doorknob, one that will open with a nail. :)