I had a friend ask how to track a car if it's stolen.
I suggested just reusing an old phone and use the built-in "where's my phone" application. For power I suggested wiring it into the car discreetly with a USB port.
If anyone's interested in a more professional car tracker, like one that's wired in, or a GPS logger, or a battery powered one that uses a magnet to attach to the underside of a car, you may want to watch this video.
Note while this can be used to track your children, it's illegal to stalk or spy on someone.