The child and the adult will both have a phone. If the child's phone (CP) leaves a defined zone the parent phone (PP) will alarm. The zone can be defined as a number of GPS points or dynamically as a distance from another phone (typically the PP). The adult can now find the child using [http://www.linuxtogo.org/gowiki/OpenMoko/ideas/GPSFriends GPSFriends]. The adult can remotely enable the microphone on the phone to listen to the child if it leaves its defined zone.
The PP will use [http://www.linuxtogo.org/gowiki/OpenMoko/ideas/GPSFriends GPSFriends] to see the location of the CP. The zone and alarming is set up on the PP and can be changed whenever needed. If the CP is outside the zone, the PP can have the CP update it's position faster than it normally would do, so it is easier to find the child.
The monitoring can be done by a proxy: E.g the parent only wants an alarm if his 2 children are too far from eachother at the funfair.
The idea or parts of it may be patented. If you know the specific patents, please update this page.
[http://www.phonetracker.de/Anwendungen/anw_personen.html An existing tracking service for cell phones]
The tracking function may also be of use in cases where the device is stolen. The device could report it's position when a pre defined number calls it. Another alternative would be calling a number automatically when an unknown sim-card is inserted. Can we read the sim-card's id information? - if not, we could checksum the phonebook on the card as an alternative.
If the thief is smart, this "protection" won't last very long -- one could always flash the firmware.