ITunes/iCloud/Find My iPhone
Have you tried setting up a family on iCloud? You can share purchases, control what they're buying, etc. Once you're in the family (or technically this works even without starting one), you can use the Find My Friends app to indefinitely share location. There's no better person to explain than Apple, so here's the link to the guide for sharing the location. https://support.apple.com/en-us/HT201087

If you're also interested in setting up the family, https://support.apple.com/en-us/HT201088

If you choose to make a family, do it before setting up Find my Friends