iMessage setup for child Apple ID
How do I set up separate iMessage for child with separate Apple ID?
iPod Touch (6th Gen)
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If your family members are 13 years old or older.
If they are under 13 years old, you will need to set up Family Sharing to create an ID for them.
Family Sharing and Apple ID for your child
When changing to the new ID, there may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion. Also set up Family Sharing to keep access to your apps/purchases.
How to transfer all files from one iCloud account to another
Apple ID email address or password - What to do after you change