Inset image thumbnail from page into list

You can use get_pages to do that:

<?php $pages = get_pages(array("child_of' => get_the_ID())); ?> 
<ul class="treatments-list h3">
    <?php foreach ($pages as $page): ?>
        <li>
            <?php echo get_the_post_thumbnail($page->ID, 'thumbnail'); ?>
            <h2>
                <a href="https://wordpress.stackexchange.com/questions/77151/<?php echo get_permalink($page->ID); ?>" title="<?php echo esc_attr($page->post_title);?>">
                    <?php echo $page->post_title; ?>
                </a>
            </h2>
        </li>
    <?php endforeach; ?>
</ul>