TV ads - actually for a battery - have run in the US recently showing a "tracking device" worn as a small bracelet or pin by a child, with "mother" able to find the child using a search device similar to a cell phone in size.

I don't recall whether the ads I've seen identified the tracking device, although they loudly tout the batteries that this particular "mother" uses in it.

It also appears that the range at which the presumedly "passive" device worn by the kid can be detected would be on the order of the "expected range of wander" for a kid at a playground or in a shopping mall within the few moments before mom would notice the child was no longer visible.

Most "chip devices," like those used for pets or livestock require the sensor to be within, at most, about a meter of the chip to read the ID.

If you have an instrument valuable enough to merit a real search, many police departments, at least in larger US cities, have "crime prevention" disignates who may be able to provide information (and local legal limits on use) for what's actually available. If the "cop shop" can't help you you may be able to find the "cop shop shop" that sells police equipment to the cops (and to persons of more vaguely defined professions), and they may - if you give them a rea$onable ju$tification - provide additional help.

John