I don't remember there being a removeable key system, but hey, if they say there's one, then all the better... The doors operate as such as any basic sliding door system. (one stationary and the other slides over it).
I am not aware of any type of locking device that could assist you.
Now knowing thier ages I would have to say again that the door is heavy and they might not have the power to open it. The handle is a big lever and is heavy to turn. (I myself sometimes struggle to open it) and the door itself weighs a lot...
You should be fine with just the removeable locking key and leaving the connecting door open to your room.