Navigation menu with children shown only for current page

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If you dont mind using a plugin I can tell you how I have gotten this to work in the past.

  1. Add the following code to your functions.php 
    function get_root_parent($page_id) {
global $wpdb;
$parent = $wpdb->get_var("SELECT post_parent FROM $wpdb->posts WHERE post_type="page" AND ID = '$page_id'");
if ($parent == 0) return $page_id;
else return get_root_parent($parent);
}
  2. Add the following to header.php between the close of head and open of body in a php wrapper 
    <?php
$GLOBALS["parentId"] = get_root_parent($post->ID);
?>
  3. Download and install the fold pages plugin: http://www.webspaceworks.com/resources/wordpress/30/
  4. Add the following code to your sidebar.php 
    <ul>
<?php
$thispage = $wp_query->post;
if ($thispage->post_parent!=0) {
        wswwpx_fold_page_list("sort_column=menu_order&child_of=".$GLOBALS["parentId"]."&title_li=&amp;child_of=".$thispage->post_parent);
      } else {
        wswwpx_fold_page_list("sort_column=menu_order&child_of=".$GLOBALS["parentId"]."&title_li=&amp;child_of=".$thispage->ID);
      }?>
    </ul>