Does anyone have a simple example of getting bestsellers from a category including subcategories?

Basically I have something like

Tools
...Tools Sub 1
...Tools Sub 2
...Tools Sub 3
...Tools Sub 4
...Tools Sub 5

and would like to create

Tools
...Tools Best Sellers
...Tools Sub 1
...Tools Sub 2
...Tools Sub 3
...Tools Sub 4
...Tools Sub 5

so the Best Sellers pulls from all the subs. Probably the top10 or 20.

All of the products in question do start with a vendor prefix 'ATEP-' so I guess I could also pull a global bestseller list and just filter for that.

Thanks.

Burt