If you're able to get the PostID, you should be able to modify your query to get all children of that PostID looking at the PostIDPath very similar to how you'd look a pages children by their alias path.
All children of About Us page: /About-Us/%
All children of post 45 in forum 1: /00000045/%
All children to a response on post 45 in forum 1: /00000045/00000068/%