Trouble adding grandchildren to a menu

I’m trying to display the grandchildren of a page, and I’ve figured out how to display the children, but I’m not sure how to integrate the code for displaying the grandchildren too. Here’s what I have so far:

<ul class="ancestor_menu">

     <?php
       foreach ( $children as $child ):
       global $wp_query;
       $postID = $wp_query->post->ID;
       if($postID === $child->ID) {
                     $active = ' class="current-menu-item"';
              };
     ?>

<li<?=$active?>><a href="<?php echo $child->guid; ?>"><?php echo $child->post_title; ?></a></li>

     <?php
       unset($active);
       endforeach;
     ?>

</ul>