List child pages with thumbnails

Look at using get_pages() along with get_children().

Did you figure this out?

I would like to create a page that lists subpages like this:
https://www.nathanlippy.com/

Try the AutoNav Graphical Navigation and Gallery Plugin.

Better still, just toss this in your parent template.

Only the thumbnails of your child pages show up, so long as you create a custom field entitled image-thumb in that (child) page with the image path it should work
.cheers.

<div class="maincontent">
               <?php the_ID(); ?>
               <?php $parent = $post->ID; ?>
<?php
query_posts('posts_per_page=5&post_type=page&post_parent='.$parent);
 while (have_posts()) : the_post();
?>

    <?php $image_thumb = get_post_meta($post->ID, 'image-thumb', true); ?>
	<img src="<?php echo $image_thumb; ?>" alt="Image" />

<?php endwhile; ?>
</div>