Have parent category contain only one post?

You can alter the posts query by using the pre_get_posts filter, and passing the parameter category__in.

In order to do that, put the following code in your theme’s functions.php:

add_action( 'pre_get_posts', 'get_parent_category_posts' );

function get_parent_category_posts($query){

   if (is_category() && is_main_query()){
       $curr_cat = get_query_var('cat');
       $query->set('category__in', array($curr_cat));
   }
}