Depending upon what iOS version that you have on the phone do you not have a Settings > iCloud > Family (iOS 10.2 or lower) or Settings > name > Family Sharing > Add Family Member option : Family Sharing and Apple ID for your child - Apple Support ?
I'm not sure there's a family sharing option on my 5c but need to set up my daughter with Apple ID so I can set her up on her 4s (wifi only no sim)
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Question: how to use my 5c to set up child apple id