Put this post-function on the sub-task workflow on transitions where the workflow could be set, typically Resolve Issue. If all sibling sub-tasks are resolved this will transition the parent issue using the provided action, which should be valid for the current step. If it’s not valid, nothing will happen.
Using the example above, this is put on the Approve transition of the Approval issue type. When all child Approval records are approved, the parent is automatically transitioned to the Approved status. Because you will often want to resolve the parent, for convenience you can specify the resolution to be used for the parent record, if necessary.