u need gps receiver and rf transceiver chips
not such an easy thing
and I dont readily see any other way of doiing it
and now that I think about it, gps doesnt have that kind of accuracy (except military)
timing radio freqs for distance determination is out of the question
signal strength might be your best bet but will effected by surroundings
ultrasound pulses are timable but gonna need a good filter to eliminate noise and a microcontroller for timing
ahh I got it !!!!
nordic semiconductor makes a multi band uhf transceiver chip that that has digitally adjustable power output and an onboard 8051 microcontroller
calibrate the output power to correspond to distance, vary the output power of the kid`s, feed back a received not-received signal from yours and display the distance.
gonna take some time
maybe the kid can help if you send him to engineering school
remember the KISS principle -(keep it simple stupid)
hope you find this helpfull
zkt