Getting Parent Categories to go full width

On my “child categories” I want for there to be a sidebar for my product filters, but for my “parent categories” I want it to go the full with of the page.

I don't list any products on parent categories, so there's no need for a sidebar and the blank space on the left of the screen looks silly, aside from being a waste of real estate.

Is there a way to modify one without affecting the other?