Glad to see you in the Community, @AnnHolly. Yes, you can create a statement billed to the parent customer with the open balance of the sub-customer in QuickBooks Online (QBO).

To include the balances of the sub-customer, you can check the Bill parent customer when setting up their profile in QBO. I'd be happy to show you the steps below:

1. Login to your QBO account and go to the Sales menu.
2. Select the Customers tab and locate the sub-customer.
3. Click on their name and select the Edit button.
4. In the Name and contact section, check the Bill parent customer.
5. Select Save.

Now, you can create a statement and send it to the parent customer. To learn more details about this process, you can check out this article: Create and send customer statements in QuickBooks Online.
 

In addition, I've also added some screenshots for your reference showing the amount and the setup.

Moreover, you can review this reference if your customer is ready to pay their balance: Take and process payments in QuickBooks Online.
 

Let me know if you need further assistance or any QuickBooks-related questions. I'm here to back you up. Take care and more sales to your business!