The Ionkids system is a PDA device with a separate wristwatch. Strap the watch to the kids wrist (or more appropriately, ankle) and know when your kids get too far away from you. The PDA allows you to set up a zone and it will alert when a corresponding watch leaves the area. Then it will show you the direction to go to recover your child. The Ionkids system can monitor up to four units at once. The system retails for $199.
Call me old school, but wouldn't a leash or a cowbell collar be more efficient? My parents seemed to think so.