I guess this might seem a little bit funny but it definitely isn't for my daughters (6 and 10). My son goes into their rooms when they arn't there and trashes their stuff, pulls things out, uses things (make up!) and loses things. I ask the girls to keep their rooms tidy but it's really hard to re-inforce this when it isn't them making a mess.
Stairgate?
My eldest has asked me for a lock on her door but I'm not keen on this.
Reward charts for behaviour don't work with him and I just don't know what to do - anyone got any advice?