No matter what you do to try and track your kids they will outsmart you, period. If they want to go racing around and drive like idiots they will do it in a friends car. If they don't want you knowing where they are they will do as others have mentioned and leave phones or the car in one location while making their way to their actual destination. That is just a fact of life. Hell if my parents had tried this on either of my cars I can tell you now their efforts would have failed.
The example previously mentioned of the kids getting hit by another car outside their own house just goes to show you will NOT be able to control the fate of your children on the road or out in the world in general. E36 M3 happens no matter what. All you can do is prepare your children as best you can to be responsible and for them to use their brains. The sad fact is no matter how much you prepare your kids there will be a percentage of teenagers that will die and there is nothing at all you can do about it unless you plan on wrapping them in bubble wrap and keeping them in a closet until they turn 20, even then you will stand the chance of them dying in some fashion other than old age. I had friends in high school that died in car wrecks, some it was because of their own idiotic driving and others it was because of the driving of others.
You also have to be prepared that doing something of this caliber is going to cause quite a bit of friction between your kids and yourself. In fact it may back fire on you and cause them to be more reckless considering you are showing them (in their eyes) that you do not trust them. When I was a teenager I trusted my father completely. I trusted in the fact that if I screwed up in the car he would take an axe to my car and then my ass would be grass.
. Now if only that tracking device would tell you if the car were airborne 
