You'll need an e-mail address for your child's Apple ID, and it can't be one you've already used for your own iTunes account. If your child doesn't have an account already, follow my
step-by-step for creating an e-mail account you can monitor, allowing you to keep track of your child's purchases and account correspondence. Select a password and security question that both you and your child will be able to remember easily. Enter your child's birthday, and choose whether or not you want your child's e-mail account to receive iTunes mailings. Then click "Continue."