How to delete a child's famili link account from your phone

1. Go to the settings
2. Find the item "Accounts and synchronization"
3. Select "Google"
4. At the very bottom click the button "More"
5. Select "Delete account"
6. Confirm by the button "Delete account"
7. Choose a parent account
8. Enter the password from the parent's account, click "NEXT"
9. Waiting for a few moments

That's all, the child's account from the current phone is deleted.