Good tracking program for iPhone 8?

My nephew recently got an iphone 8 for Christmas, his mum activated the iphone built in tracking program so she could keep an watch that he wasn't wandering too far without permission, unfortunately the built in tracker only work when there's a Wi-Fi connection and many places my nephew and his friends go do not have free Wi-Fi, is there another iphone tracker app that doesn't rely on Wi-Fi and will use gps or something - she is willing to pay for a good app

There's no app out there that could track your mom's son without Wi-Fi or cellular service. Maybe you guys can buy him a cheap portable hotspot and he can bring it with him whenever he leaves so his phone has has Wi-Fi. If you guys don't want to spend the money on something like that, then maybe just give him a curfew and tell him where he can't go. You guys could just trust that he'd stay close to home.

If by "built in tracking program" you mean Find My iPhone, that's not its intended purpose. There's one called Find My Friends which is more suited for that sort of thing. In any case neither of those apps only work through Wi-Fi. They work through data connections as well. If he doesn't have a data connection then there's not much that could be done there.

Apps like those need internet to be able to place the person in a location. GPS will always work on the phone itself but requires internet to be able make the location usable/broadcasted.