How can I show the subcategories content on the parent category page?

One possibility:

<?php 
    $sub_cats = get_categories('parent=" . get_query_var("cat'));
    if( $sub_cats ) :
        foreach( $sub_cats as $sub_cat ) :
            $sub_query = new WP_Query( array(
                'category__in' => array( $sub_cat->term_id ),
                'posts_per_page' => -1)
            );
            if ( $sub_query->have_posts() ) :
                while( $sub_query->have_posts() ) : $sub_query->the_post();
                    //your output//
                endwhile;
            endif;
        endforeach;
    endif;
?>

http://codex.wordpress.org/Function_Reference/get_categories
http://codex.wordpress.org/Class_Reference/WP_Query