How do I set up my iphone to track my child's iphone

Share your location with iPhone - Apple Support

Find friends and share your location with Find My - Apple Support

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

It sounds like you could set up your child's iPhone to share its location with yours, one way or another.

"If you already set up Family Sharing and use Location Sharing, your family members automatically appear in Find My."

I didn't skim the articles thoroughly enough to see if there was a way to lock the Find My settings, to prevent your child from turning them off.

Note that Find My relies on the Internet, so when your child doesn't have an Internet connection, you probably will not get location updates from their phone. (There's the Find My Network, but that relies on someone else with an Apple device coming within very close range of the phone you're trying to locate.)

on your child's iPhone go to find my - people - and share their location with yourself.