All child categories on parent page

In WordPress I have a menu like this;

1Parent category
-Sub category
-Sub category
  *Sub sub category 
  *Sub sub category
     **Product

2Parent category 2
-Products

I want the following thing;

When I click on a Parent category, that page has to show in the CONTENT all his sub categories (children). When a parent category has no sub category he needs to show the posts that are assigned to that category. This also with sub categories that has a sub (children).

Other explanation :
When I visit the website I need to choose a main category. When I click that category he needs to show the sub categories, when I click the sub he needs to show his products. Or when a main category has no subs he needs to show his products.

I hope you guys understand it and that you now a solutions. Thanks!!