One issue is that kids are a lot more willing to be without this dedicated tracker than their phone.
As long as they can get the wristwatch back before going home, there's no cost to leaving the wristwatch where they're supposed to be and then continuing with their regularly scheduled mischief. But kids hate to be separated from their phone.
Now, it's possible that it could have some combination of sensors that at least made pretty sure that some humanoid was wearing the thing. But that increases the cost and battery usage.
Another issue is that the kids could just "forget" to charge it a lot.
Integrating it with their phone means that most things that keep the tracking from working also keeps them from using their phone, which is pretty much a nightmare to most teens.
