#1MinuteTip You can now use the External Account Hierarchy to share records owned by the users of child account with users of the parent account. 

Just enable the External Account Hierarchies in Communities Settings. After you enable the preference. the External Account Hierarchy object is available in your org.

external account hierarchy

Reference URL: Click here

P.S.: You may also want to check out my other blog post on all the different Ways to Share Records in Salesforce