Requirement: To get all Sales Orders created by all the sub-customers on parent customer as a sublist.
As per Netsuite’s standard functionality, the customer record shows all the transactions created by that customer only.
Solutions:
Step 1: Create a Saved Search on Customer record
A) Set Criteria:
1. Type: is Sales Order
2. Main Line: is True

B) Set Results:
Add fields as shown below figure:

C) Set Available Filters:
Add Field as shown below.

Now Save the Saved Search.
Step 2:
1.Go To Customization -> Forms -> Sublists as below.

Now click Entity Tab and select the saved search that you have created just now. Add the tab on which you want to show and select Customer checkbox as shown below.

Now add and save.
Now Go to the Customer record -> Sales -> Cost Center Transactions and see the results.
