Hi,
When you create a test account, it will be in canceled status by default. To see the cancelled accounts from the UI, you will need to adjust the filters in the top left corner. When you run $customerService->getCustomers(), the API will only return the accounts on which you are a user. In your case, if you are not a user on the client accounts, only the MCC accounts will be returned. To get the account hierarchy, you will need to use the ManagedCustomerService.get().
Regards,
Dhanya, Adwords APi Team