Hey mark, good news - the iPhone actually stores this natively! Go to Settings > Privacy & Security > Location Services > System Services > Significant Locations - it keeps a history of places visited, though you’ll need Face ID/passcode to get in.
If you want something more real-time and easier to check regularly, just set up Family Sharing with Find My - it’s free, built into Apple, and you can see where they are anytime from your own phone without touching theirs.
For most parents that combo is honestly all you need without paying for any third-party app.