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>