Modify Products Filter

I have 3 (parent)categories, each with 4 (children)subcategories. I modified my shop blade so that there are 3 <input> lists where the user can select only the children subcategories (no parent categories). Right now, all products belonging to each subcategory are shown, but I'd like to modify my Filter option so that the products shown must be within all the children categories selected. Suggestions are kindly appreciated, thanks in advance.

Example: There are two parent categories named: Country=[Spain, Portugal] and Produce=[Wine,Oil, Ham]. Selecting "Spain" and "Wine" only returns Spanish Wines, not all products in Spain and all products in Wine. Additionally, only if children belong to the same parent category then the all results are added (e.g. selecting Spain and Wine,Oil then all wines and all oils from Spain are shown).