In preparation for my fiancee and her daughter moving in next month, I installed a key-pad block on the door to Youngest Son's room (which will be the daughter's room when she moves in). It eliminates Middle Son's ability to go into the room and get into things that he shouldn't. Currently the only issue is that the goes in and takes Youngest's shirts and wears them as pants. Irritating, but not serious. When the daughter moves in, there will be make-up, nail polish, and other things in the room that are potentially dangerous.
I wasn't sure how Youngest would react. It turned out that he loved it. He kept coming to me and telling me how he'd pushed the buttons to lock and unlock his room.