Here is a hi-tech “kid leash” that consists of a transmitter which you attach to your child, and a receiver which is carried by an adult. The transmitter emits a signal that is picked up by the receiver which determines how far away your child has wandered. You set a distance, and if the child wanders away greater than that distance, an alarm sounds.

“In terms of range settings the Kiddo receiver actually only has 2 options. The short range setting will alert the adult if the child has gone more than 4 meters (about 13 feet) away while the long range setting will sound the alert when a distance of 8 meters (or 26 feet) has been reached. The transmitter is also waterproof which means it’s pool and beach-friendly and can even be used at airports and on airplanes without disturbing the avionics.”

via ohgizmo.com