Actually, there are pros and cons with both tracking methods. GPS needs direct line of sight but cellular can work indoors too. If we use GPS and child is kidnapped and kept indoors, we can’t track the child. Plus there needs to be an additional method of communication.
If we use cellular for communication, all we need is an active mobile number (SIM). The system should be designed such that, when a coded SMS is sent to it, it should reply back to the server with the Cell Broadcast or cell tower number. This will give us an approximate location. Triangulation can be done by the police, who should be definitely involved in when a child is lost.