I saw some "childproof" locks a while back, I believe in rockler, that used a magnetic key to open. The hardware was mounted on the inside of the door, and no key hole was needed, instead, a magnetic key was provided, which you placed over the lock assembly from the other side of the door, and you could then open the door. I'm not sure if that will help you, but when I saw it, I thought it might be useful for locking a hidden door. I'd ask them what they might want on that wall in their bedroom that could be used as a door- Tv cabinet, mirror, shelves, ect. If there is something to cover up the opening, it's a lot easier than making the door blend into the wall.
Edit- Here is the lock I was talking about
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http://www.kidsafeinc.com/product/71...&utm_campaign=