Customers can manage their children's invoices through the SSP.
The customer can view their current subscriptions. The subscription section also indicates who will be invoiced for the subscription.
In the Invoice section, you can sort to view the customer invoices as well as any children invoices. This children's view of invoices includes unified invoices from the child customer's children. The parent can pay for any invoice that a child is paying by clicking Details next to the invoice and then clicking Pay. The parent customer can pay for a child invoice using the parents's payment methods.