High security in the kid's room

My 7 year-old son has always had a strong interest in "keep out" signs for his room to keep his younger sisters from interrupting his Lego work. First there was crayon on the door ("kep owt!), followed by scotch-taped signs including dire warnings to those who dare enter and a carefully drawn skull-and-crossbones.

Now he's taken it to another level with something my wife gave him: a keycard activated alarm! He swipes his card when he leaves the room. If anyone opens the door without swiping a card, they get a recorded voice saying "Keep out!" followed by a brief siren alarm. It's a hoot! And he absolutely loves it.

It comes with two keycards: one for him, and one marked "guest" for mom and dad--emergency use only.