Add link to parent page in list of child pages

you would add something like

//* List child pages [jla_childpages]
function jla_list_child_pages() {
  global $post;

    $parentID = ( is_page() && $post->post_parent ) ? $post->post_parent : $post->ID;
    $parentPost = get_post($parentID);
    $isParentCurrent = (get_the_ID() == $parentID) ? " current_page_item" : '';

    $parent = "<li class="page_item page-item-{$parentID}{$isParentCurrent}"><a href="".get_permalink( $parentID )."">{$parentPost->post_title}</a></li>";

    $childpages = wp_list_pages( "sort_column=menu_order&title_li=&child_of={$parentID}&echo=0" );

    if ( $childpages )
      return '<ul id="childpages-menu">' . $parent . $childpages . '</ul>';

}
add_shortcode('jla_childpages', 'jla_list_child_pages');

I haven’t tested it, but all you’re doing is figuring out the parent before everything, and including the parent above the children in the return.