Ph.D. in electronics needed.
The word is "ping". It's an intransitive verb in the English language.
The process is useful when standing outside the restroom for the opposite gender while your child is inside, and can be adapted for a variety of child-parent separation situations.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm
"There are no lost children at WDW, only lost parents."
While it is possible to "lock down" the turnstiles, how practical would that be on a busy day with people rushing to catch Magical Express buses and the like? And would such a procedure be foolproof? Would there be cadres of CM's, in number comparable to who greeted the family who won the hour to themselves in the Magic Kingdom, to sweep-search every restroom and nook and cranny of the park?
Probably the closest to foolproof system to prevent abduction of children from WDW is an electronic ID, a grain of rice pellet injected into every person's arm. Many oppose such a nationwide system but a lost parent alert sounded in a park could then be followed up with the scanning of every child exiting the turnstiles with almost no inconvenience or delay.