Only filter in one ( on sale ) category

Hi there,

I’m looking for a way to filter between child categories of the ‘brands’ category in a dropwdown ( ‘Brands’ is a parent category ) but then only show the items of that specific child category, that are also in the ‘sale’ category. Is that possible?

So like this: https://finds.nl/winkel/
But then only with “sale” items showing up.

I created a Sale testing page here: https://finds.nl/sale-test/

Any idea? cant seem to find a solution. Hoping for some help! Thank you.

Grts

The page I need help with: [log in to see the link]