How do you manage Budgets for Programs in Budget Control environment?

In a Budgetary Controlled environment (Non-Integrated with GL Budgets) and when you have a Program linked to multiple Child Projects, where do you enter and maintain the Budgets?  Do you maintain Budgets at the Child Project Level and it rolls up to the Program?  If so then how and for what purpose?

When I create a scenario as explained above, I try to create a Purchasing Requisition and enter the POET for the Program but it tells me there is no Budget for the project; because I did not create one since I expected the Child Project Budgets to roll up and be available at the Program level.