Cannot start family sharing: Apple id is already used for shared purchases
I'm trying to set-up family sharing on multiple devices, but I'm informed that my mac.com address is "already used for shared purchases" and "accounts can only be part of one Family at a time" with a dialogue box of "OK". We are only one family, and all devices were recently under my iTunes/Apple account. How do I change that so that I can set up family sharing? I have an 11 year old itching for her own phone and want to be able to share, but also to manually approve purchases (which I think is the point of family sharing, especially with someone under 13.)
iPhone 5s, iOS 8