List_parent_Page_and_all_descendant_Pages.php

    <?php 
// 指定したIDの子孫を階層でリスト表示
$parent = ここに親のID;
$args=array(
  'child_of' => $parent
);
$pages = get_pages($args);  
if ($pages) {
  $pageids = array();
  foreach ($pages as $page) {
    $pageids[]= $page->ID;
  }

  $args=array(
    'title_li' => '',
    'include' =>  $parent . ',' . implode(",", $pageids)
  );
  wp_list_pages($args);
}
?>