As far as the anti-kidnapping device, it would work a bit like an EPIRB.
GPS devices need only RECEIVE a signal from GPS satellites. A feeble signal that is readily blocked by moving inside a building or even the back of a van with no windows, or a car trunk.
A kidnap-detector device, like an ankle bracelet would only need to communicate as far as the local cellular network. However, cellular devices can readily be jammed by off-the-shelf jammers, and naturally won’t work if you’re in an area not covered by the cellular network.
A single-use lithium button battery could run the device, probably strobe last known location to the nearest cellular tower every minute for maybe up to an hour. That’s a tiny window of opportunity for police to react, organize a chase and do something about the kidnapping. Kidnapping gangs would quickly adopt countermeasures.
Cellular jamming, faraday cages for transporting the kid, etc. Of course, the most natural countermeasure would be searching for the device and cutting it off. If the device is too tough, then the hand or foot, arm or leg below the device would have to do. If implanted, it would still leave a lump and would necessarily still be directly under the skin. Kidnappers would search for those lumps and cut them out or sever the whole limb.
These kidnappers, members of drug gangs fleeing or pursuing murder, rapists, child rapists, etc. are among the people who illegally cross our borders among the economic refugees every day. Many already fleeing Mexican justice. THEY are why ‘illegals’ have such a bad name. Maybe one in a thousand are criminals, but that’s plenty enough trouble to poison relations.