As QuickTimeKirk posted, get them their own ID for use on their phones. 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.
If they are 13 years old or older.
Apple ID – Create
If they are under 13 years old.
Family Sharing and Apple ID for your child
How to transfer all files from one iCloud account to another
Apple ID email address or password - What to do after you change
Family Sharing
Family Sharing - Set up
Family Sharing - Use your own Apple ID for Family Sharing
Family Sharing -What types of content can I share?
Family Sharing - If you don‘t see your family‘s shared content
Family Sharing - purchases and payments
Family Sharing - Hide and unhide music, movies, TV shows, audiobooks, and books
Family Sharing – share iCloud Storage
Family Sharing - Share an iCloud storage plan with your family
Family Sharing and location
Family Sharing - Ask to Buy
Family Sharing - Ask to Buy - Get Help