In Salesforce, if the organization-wide default for Accounts is set to Private, access to child accounts is not automatically granted based on parent account ownership. As a result, manual sharing rules or sharing through Account Teams or role hierarchies must be used to extend access to child accounts. The Account Hierarchy itself does not provide automatic access to child records.
Account Teams can provide access but only if configured to include child accounts. Since the OWD is Private, manual sharing is the primary method to ensure appropriate access.
Salesforce Documentation Reference:
Account Hierarchy and Sharing
Manual Account Sharing