Seperating phones from the same Apple id

You won't be able to do that. If you want to separate from the family ID, you'll need to create a new one.

See How to create a new Apple ID https://support.apple.com/en-us/HT204316. See the data implications below.

Some of these things may apply even though not changing an Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

 Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.

And see what Eric Root says on this matter here https://discussions.apple.com/thread/251343949 and

what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747

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You also can set up family sharing between the IDs to share purchases made with them. See:

Family Sharing  https://support.apple.com/en-us/HT201060

Set up Family Sharing https://support.apple.com/en-us/HT201088

If under 13 years old, you will need to set up Family Sharing to create an ID.

Family Sharing and Apple ID for your child.