Standard Posts Module to Display Category's Sub-Categories (Child)

I’m trying to use a Standard Post Module for my website to display different forums and their corresponding discussions within those forums. When I add the post module to a page, and go under Content, I set the Source to Custom Query and then Post Type I would like to be Discussions (sub-category of Forum category). But, in this, I don’t want all the discussions from all the other forums (parent category) to show, just that one forum’s discussions.

Is there a way to show ALL sub-category posts within the parent category without filtering and matching every single discussion (we have about 8 forums and discussions are added everyday to each, so I don’t want to have to update this module to explicitly include a newly created post every time one is created)?