SELECT customer_client.descriptive_name FROM customer_clientHi Benjamin,
Thank you for reaching out. Please follow this example for getting all the client accounts under an MCC account. If that doesn't work, please provide the complete request and response logs via reply privately to author. I'm not sure which client library you're using, but here is a guide on how to enable logging in the Java client library.
Regards,
Mitchell
Google Ads API Team