To change who is responsible for billing, follow the screen shot below.
Once you change who is responsible for billing, when you go to receive money while looking at a child account, the parent account's payment information (Credit Card or ACH accounts) won't be available from the drop down. It will now show the child account's payment information (Credit Card or ACH) if they are on file.
Invoices will now go to the child account once jobs are completed. The account balance will now also show on the child account instead of on the parent account.
If the parent account is responsible for billing, it won't show this balance on the child account and it will have this blue indicator showing that the parent account is responsible.
Where it can get really tricky is if the child account has multiple contracts. If you have changed the responsibility for billing to the parent account but you still see a balance on the child account, the child account most likely has multiple contracts. If you want the parent account to be responsible for all contracts for the child account, you will need to assign them all to the parent account.