All post of child category not in top category

Maybe something like this would do?

  $cat="About us";
  $tax = 'category';
  $cat_id = get_cat_ID($cat);
  $category = get_term_children($cat_id, $tax);
  $pre="";
  foreach ($category as $sub)
  {
      $cat_list .= $pre . $sub;
      $pre=", ";
  }
  query_posts(array( 'cat' => $cat_list, 'posts_per_page' => 1000));
  echo '<ul>';
  while ( have_posts() ) : the_post();
    echo '<li>';
    the_title();
    echo '</li>';
  endwhile;
  echo '</ul>';