Log-out from the child’s account on the device
When logged in to your child’s account in a browser, go to https://myaccount.google.com/. Security -> your devices -> pick the required device -> click on ‘sign out’.
Remove the child’s account from the device
On the device, remove your child’s account from the device:
- Open the Settings app
- Tap Passwords & Accounts
- If "Passwords & Accounts" isn't available, tap one of the following depending on your device:
- Users & Accounts
- Passwords, passkeys & autofill
- Passwords, passkeys & accounts
- Manage accounts
- Under "Accounts," tap the account you want to remove, then Remove account
- To confirm, tap Remove account, then Delete
- If this is the only Google account on the device, you'll need to enter your pattern, PIN, or password for security.
Remove the ‘Family Link parental controls’ app
Now that the device isn’t managed by Family Link, you can remove this app.
Factory-reset
Your Android device can now be reset to its factory settings.
Copied from https://noamlerner.com/posts/factory_reset_android_device_with_family_link/