Global Sub Categories

Hi, I have just switched over to this theme, for simplicity. I have used a text widget to add global categories to the sidebar, as I don't want to use every category that members put up on their blogs. Now I need to add sub-categories, and I was just wondering would there be a way to change the widget to one that displayed the sub-categories when on a category page? Would it have to be coded into the sidebar.php?

Thanks.