I finally caved and allowed my 16 year old daughter set up a separate iCloud/iTunes account because of all the problems we were having when using the same account. However, now I can no longer see her phone on Find My iPhone. I think when she set this new account up, it turned it off. I had her turn it back on, but I cannot see it. Any advice would be most appreciated. I need to be able to have this capability. I have a 12 year old and we are also having issues with using the same iCloud account, but I won't change his until I have a solution.
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