Set up old phone for a kid
How do I re register my old phone so it is in my son's Apple ID
Before you sell, give away, or trade in your device, you should remove your personal information. You shouldn't manually delete your contacts, calendars, reminders, documents, photos, or any other iCloud information while you're signed in to iCloud with your Apple ID. This would delete your content from the iCloud servers and any of your devices signed in to iCloud.
see this link:
https://support.apple.com/en-us/HT201351
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