Q: How can I add a child to family sharing if they already have an iCloud email address? I get a message saying the Apple id is already being used.

Switched from iPhone 5 to a new iPhone 6. My family had always shared my wife's apple id since she had an iphone prior to me getting my iphone 5 and daughter getting 5th gen iPod.

We now want to do Family Sharing.  I created my own Apple ID and invited my wife to join.  No problem.  Now I want to add my 10 year old to our sharing.  She already has an icloud email address, but when I type that in, it tells me that my wife is already joined (obviously daughter's email is associated with wife's apple id). 

When I try to "create" an apple id for my child and type in her email, I am told "This Apple ID is already being used.  If this is your account, reset a forgotten password at https:forgot.apple.com."

I would like to be able to add my child to our Family Sharing, but don't want her to have to change her email address as it will affect her contacts with friends/family.  How can I do this?