We are in the middle of transitioning to our new system. Under the new system users must be logged in to purchase a child theme without Genesis. This helps stop first time customers from purchasing just the child theme, which can cause them a lot of frustration.
Emails are going out right now so watch your inbox from the account you used to make the purchase. The login will allow you to access the new system which is an all in one portal for support, tutorials, and downloads. It will also allow you to purchase the child theme only.
Thanks for your understanding and patience during this transition.