Hello Miguel,
With the Oauth credentials of the Top level manager account you should be able create the client accounts for his client. But the Client customer id in you call should be of the child manager account. That says you can make API calls from the top level manager account on behalf of its childs. Using an approved developer token, you can make an API call to any Google Ads account.
Regards,
Sai Teja, Google Ads API Team

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