Note: if you are a host, looking to add a child for a customer of yours, use this article.
As a customer, when it comes to adding a child account to your account, you can go one of two ways:
use an existing email address that belongs to your child
use an alias (your email with a +1 appended)
This second option requires the email client supports alias functionality (for example, aol and yahoo currently do not. Gmail does.)
To get started, login as a customer, click My Children > Add child Account. After an account has been created, you or the child will need to confirm the account by email before you can book for them on a checkout page.