Let me assist you further, @dannaFlorius, and help you in running a report that'll show the due dates of invoices and when it was paid.

I've read the thread and replicated your goal and found out that we can run the Invoices and Received Payments report in QuickBooks Online (QBO). However, we need to customise it first to view the desired data.

I've created a Parent customer and a Sub-customer. I've recorded the invoice under the Sub-customer and the payment is under the Parent customer. When running the report, the names of both Parent and Sub-customers show. 

See the attached screenshot below. Sy Noe is the Sub-customer and Jio Van is the Parent customer:

Also, as per the screenshot provided, we're able to see the Due Date of the invoice and when it was paid. This can be possible by customising the report:

1. From the Invoices and Received Payments report page, click on the little gear icon then hit on the Show More dropdown.
2. Mark the Due Date box from the list of filters.
3. When done, click on Run report.

Additionally, in case you want to save the report's customisation so you won't have to repeat the process over again, feel free to browse through this article: Save custom reports in QuickBooks Online.

I'll be around in case you need further assistance with regard to accessing reports in QBO. Simply leave a comment below, and I'll get back to you. Keep safe.