The Room Defender is a $33 tripod-mounted, foam disc-shooting weapon that has a motion sensor to detect intruders into your kid's overly-moist room. When someone enters, the Defender gives them a verbal warning, then proceeds to pelt them in annoying discs until it runs out (depending on setting). There is a remote which can be used to control and disarm the gun, when necessary, although it doesn't appear that the unit actually tracks its prey, meaning a single well-placed dive roll would probably put you out of harm's way. Alternately, burn your child's room or your child into cinders.
Update: Target sells it for $20 here in the US.
Catalog Page [Target]