This tech. doesn't track the kids
It only tracks the phones - and since it uses GPS, it doesn't work if they're indoors, or switch it off.
Presumably, any savvy kid could leave their phone and therefore the GPS, on when they go to a "good" place. Then turn it off and nip over to their mate's house and once inside, switch it back on again. The GPS would only record the last known position - i.e. when they went into their house. After the phone is indoors it loses the GPS signal. When it's off it can't know it's been moved, so when it is switched back on, then provided it's indoors again the GPS can't say that it's not in the same location anymore.
That's assuming the kids don't just leave the phone switched off - or even go out without it.