"Have you ever experienced the terror of thinking your child is missing or lost? We have all had the gut-wrenching feeling of turning around in a busy street and finding that our child is nowhere in sight. Now this can be prevented with the new and improved Child Guard. The electronic child leash lets you know if your child roams further that you're comfortable with.
The child monitor signal is adjustable - from a distance of 6 to 30 feet, so when the child goes beyond the set distance, the adult's receiver starts to beep..."
~Thanks Diederick who points out "the child leash" description of this device.
Also note that a maximum distance of thirty feet might limit it's usefulness to infants and toddlers.