Show only subcategories from only one Parent category

This is what I’m trying to figure out.

I’m planing to have 2 navigation menus, one just above the fold and one on the sidebar of the page.

Now, I have a parent directory called ‘Authors’ and lots of sub-categories with authors’ names (ex. Tony, Rohn etc.). There are also other categories like ‘Poems’, ‘Sonets’ etc.

So far so good. Now the problem is that I want that on the fold menu there would be shown only sub-categories from ‘Authors’ parent category, hiding all the other categories. And on the sidebar there would be shown all the categories except the ‘Authors’.

Anyone has an idea how to do this? :\

Thanks in advance, guys!