Could active or passive radio frequency identification technology be used for this purpose?
Radio frequency identification cannot be used to detect a lost child. RFID is a relatively short-range technology used for identifying, tracking and managing objects or individuals. Passive RFID transponders can communicate with readers at distances ranging from a few millimeters to 30 meters or so. Active tags have a read range of several thousand meters. If you were to put a passive or active tag on a child, and if he or she were to become lost at a shopping mall, you would need to install readers throughout the mall to track that child. If the child were kidnapped, RFID would be of no use whatsoever.
There are Bluetooth-based systems available on the market that can alert parents via their smartphone if their child strays too far. That's a better solution for this application than an active or passive RFID system.
—Mark Roberti, Founder and Editor, RFID Journal