I'd like to View Subcategories on Inventory Page with clicking into the category first

Hello, We have two categories on our inventory page "New Inventory" and "Pre-Owned Inventory". Under each category we have 5-10 subcategories. I'd like the subcategories of both categories to automatically show when someone goes to our inventory page. I've tried a few custom ccs code sequences and can't get one to work. I'd like to keep the menu on the left side as well. 

Any help is appreciated! 🙂