Well, it kinda works like this. If you look at our demo site and click on the filters you'll see that when you click on a parent category the sub categories are automatically selected. So, I've made it easy for the end user to select partent and sub cateogries.
Initially I thought to implement it so that when the parent category is selected in the background we load everything that is in this category. From one side that is logical, but on the other side it creates performanced problems where I have to figure out if the current category has children etc. And there were other situations where it wasn't logical. Let say that you have selected the parent category and one children category - wouldn't you expect to see only stuff from the parent and that children category? I would, but for some reason the system would return stuff from all subcateogries... With the current implementation where the user has to explicitly select what he wants we solve this issues. And the automatic selection of children makes it easy for the end user.
So I guess that you'll have to go in the menus and select exactly what you want to show
How often do you add new categories?
Regards,
Daniel