I used this code to loop through child pages of a custom post type called work. Each Work piece had a title, featured image and pdf. The child pages, describe the details of the project or work piece and display as an accordion. So basically what we have is an accordion within an accordion.

chilpages-looping.php

<?php 

// loop through the sub-pages of your custom post type
$childpages = new WP_Query( array(
	'post_type'      => 'work', 
	'post_parent'    => $this_page,
	'posts_per_page' => 100,
	'orderby'        => 'menu_order'
)); 
	while ( $childpages->have_posts() ) : $childpages->the_post(); ?>

	<section>
			
		<h2 class="title"><a href="#"><?php the_title(); ?></a></h2>
			<div class="content">
				<?php the_content();?>

				<?php $this_subpage=$post->ID; ?>

				<?php 
				//Loop through the sub-pages of the child pages next
				$subpages = new WP_Query( array(
					'post_type' => 'work', 
					'post_parent' => $this_subpage,
					'posts_per_page' => -1,
					'orderby' => 'menu_order'

				));

				while ( $subpages->have_posts() ) : $subpages->the_post(); ?>
					<section class="subpages">

						<h3><a href="#"><?php the_title(); ?></a></h3>
						<div>
							<?php the_content();?>
						</div>
					</section>
				<?php endwhile; wp_reset_query(); ?>

		</div><!--.content -->
	</section>		
<?php endwhile; wp_reset_query(); ?>

Where do you define $this_page?

You sir, are awesome and I love you!