if($post->post_parent)

                      $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");

                      else

                      $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");

                      if ($children) { ?>

                      <ul class="sub-menu">

                      <?php echo $children; ?>