You cannot change who this customer bills with because ... Delete all payments or invoices to this customer's parent ... Spoke w/ 3 people and no one was able to help.

Hello, @sarahi.wilson.

I hope you're having a great week so far. 

Your question has been responded to on this thread: How do I change a parent of a customer?...  The article mentioned will help you determine how to bill the customer and sub customer on the same Invoice. You'll also want to refer to the screenshot provided. 

Please let me know if you need any additional information. Enjoy the rest of your week.