Do I (or the child) need to have an existing ShareBuilder account?
No. The process of becoming a custodian creates a ShareBuilder Custodial Account in your name as custodian for the child you specify. No other accounts are necessary. When the child reaches the age of distribution (which varies by State), the assets of the custodial account become the direct property of the young adult. Then a standard ShareBuilder account can be opened and the assets transferred without charge.