I should note that over the years, due primarily to the complications of account security, we no longer split accounts. If a minor that you are the parent or legal guardian of plays on your account, which they are still allowed to do, you would need to use a character service to move any character to their own account (assuming you share a last name).
It is usually best now to either create an account in their name if they are over 13 but under 18 (with parental controls), or if they are under 13, create the account in your name which can be updated when they are of age.