Fully understand your demand and idea.
Members relations and corresponding rights are part of the Community Builder 2.0 development that is taking place, and I'm expecting them to solve this and many other use cases.
In the mean time, if you are using Joomla 1.7, I could imagine that using SQL queries integrations to add/remove groups from children when parents pay, and Joomla ACL groups for rights extensively, something basic suiting your needs could be achieved already with CB 1.7 and CBSubs 1.2.2.