AirTags rely on being able to piggyback off of other iPhones to report their location. They are not a self-contained device, merely an appliance. If you give your kid a smartwatch with cellular or a full-fledged phone, you can track where they are but also they have the ability to contact you/be contacted if need be. Sure, there are drawbacks to such choices, but that is what being well-informed as a parent is all about.