get-grandchildren.php

<?php
  $parentId = get_the_ID();
  $args = array(
    'post_type' => 'page',
    'posts_per_page' => -1,
    'post_parent' => $parentId,
    'orderby' => 'menu_order',
    'order' => 'ASC'
  );
  $posts = get_posts($args);
  if ($posts):
    foreach ($posts as $post):
      $grandchildren = get_page_children($post->ID, $posts);
      if ($grandchildren):
        foreach ($grandchildren as $grandchild):
?>
<li class="grandchild-link">
  <a href="<?php get_permalink($grandchild->ID)?>"><?php $grandchild->post_title;?></a><!-- 記事タイトル表示 -->
</li>
<?php
        endforeach;
      endif;
    endforeach;
  endif;
?>