How to display products only from first child category on category page?

[powercoders]

I have Virtuemart 3.8. Trying to display products only from the first child category on category page.

For example we have:
- Category 1
-- Category 1.1
-- Category 1.2

On "Category 1" page need to display products only from "Category 1.1".

Tried to edit /components/com_virtuemart/sublayout/products.php, /components/com_virtuemart/views/category/tmpl/default.php but have no result. Thank you in advance!