Show children of child page within a query

I am trying to find a way to show a first level page’s children’s title, thumbnail and a few custom fields. I have been able to successfully show the children of the first level (send level). Where I am getting stuck though is that within each child page I’d like to show it’s children as well (third level). I fist tried setting this up using $wpdb->get_results however it seems like after looking at other support forums it looks like I should be using WP_Query instead.

If anyone can help point me in the right direction it would be greatly appreciated. Below are the two options I’ve tried.

   <?php
// The Query
$the_query = new WP_Query( array(
    'post_type' => 'page',
    'post_parent' => $post->ID,
    'posts_per_page' => -1,
    'orderby' => 'title',
    'order' => 'ASC'
) );

// The Loop

 if ( $the_query->have_posts() ) while ( $the_query->have_posts() ) : $the_query->the_post(); 
 		$thumbnail = get_the_post_thumbnail('sample-thumbnails');
		$postexcerpt = get_the_excerpt();
		?>
        
  <div class="sub-container group">
      
<div class="thumbnail-wrap t-1of2 d-1of2"><?php the_post_thumbnail('thumbnail') ?></div>
 <div class="sub-content t-1of2 d-1of2 last-col"><?php the_title(); ?>

<p><?= $postexcerpt ?></p>
 <!--list 3rd level pages --> 
           <?php $granchildren = get_page_children($page->ID, $pages); ?>
          <?php foreach ($granchildren as $grandchild): ?>
         <h3><?php echo $grandchild->post_title; ?></h3>
          <?php endforeach; ?>
          
          </div>
 </div>  
<?php endwhile; ?>

<?
$child_pages = $wpdb->get_results("SELECT * FROM $wpdb->posts 
WHERE post_parent = ".$post->ID." 
AND post_type = 'page'
AND post_status = 'publish' 
ORDER BY menu_order", 'OBJECT');

if ( $child_pages ) :
    foreach ( $child_pages as $pageChild ) :
        setup_postdata( $pageChild );
        $thumbnail = get_the_post_thumbnail($pageChild->ID, 'sample-thumbnails');
		$postexcerpt = get_the_excerpt($pageChild->ID);
		?>
        
              
      <div class="sub-container group">
      
        <div class="thumbnail-wrap t-1of2 d-1of2">
        <?php if( $caption ): ?>
         <img src="<?php echo $thumbnail['url']; ?>" alt="<?php echo $thumbnail['alt']; ?>" />
         <?php else:?>
         <img src="<?php bloginfo('template_url'); ?>/images/default.png" width="490" height="345" alt="Image to come"/>
<? endif;?>
        </div>
        <div class="sub-content t-1of2 d-1of2 last-col">
          <h1><a href="<?= get_permalink($pageChild->ID) ?>" rel="bookmark" title="<?= $pageChild->post_title ?>"><?= $pageChild->post_title ?></a></h1>
          <p><?= $postexcerpt ?></p>
           
           <!--list 3rd level pages --> 
          <?php $granchildren = get_page_children($pageChild->ID, $pages); ?>
          <?php foreach ($granchildren as $grandchild): ?>
         <h3><?php echo $grandchild->post_title; ?></h3>
          <?php endforeach; ?>
          
          
        </div>
            
            
        </div>
        
      <? endforeach; endif;?>