Searching for this Custom Loop

Hello guys,

Situation : the Archive page template.

The problem : a loop retrieving posts from the Main, Parent Cat, but not the posts that are deeperly categorized (instead, reserve the sidebar for them)

I have category "A", "B" and "C"

"A" have the sub-categories "a"+"aa"+"aaa", "B" have the sub-categories "b"+"bb"+"bbb", and "C" have the sub-categories "c"+"cc"+"ccc".

How could I set the Archive page main loop showing just posts in "A" , letting alone those that have deeper categoritzation ? Could I at the same time show in the sidebar the lastest posts which are marked in those deeper levels ? Say 5 titles for subcategory ?

<h2>Sidebar</h2>
And in the "a" Subcat ... :
- tile 1
- tile 2
- title 3

How would you do it ?

Thanks in advance.