display posts from specific category on a page , in thematic child theme?

Try changing :

$wp_query->query( array( 'posts_per_page' => get_option( 'posts_per_page' ), 'paged' => $paged ) );

to

$wp_query->query( array( 'category_name' => 'news','posts_per_page' => get_option( 'posts_per_page' ), 'paged' => $paged ) );

You need to use slug, in this example “news” is the slug of the category.