There are a few ways of doing it:
You can use the api below and get all the courses from a account.
/api/v1/accounts/:account_id/courses GET API https://canvas.instructure.com/doc/api/accounts.html#method.accounts.courses_api
You can use the canvas data portal and get the course_dim/fact and account_dim/fact tables (need sql basic knowledge - select/joins)
You can set a sis_id in all courses from that sub-account. Than, probably, all the courses will show into the admin report.
Hope it helps!