Chances that a child is abducted are marginal - chances that it lends its phone/device to a friend, has it stolen, or places it at a inconspicuous place while going on a romp are high indeed. That is, short of GPS implants (which is just one step off brain surgery to make children more docile, IMHO - welcome to our brave new world) nothing will enable you to eavesdrop on even marginally smart children, and with the gap in technology competence between teenage children and their parents you should better start to worry when your kids will plant a GPS bug on you rather than vice versa...