List child pages and featured image

I’m using this string of code to fetch a custom post type (Sample), as well as it’s featured image:

<?php
global $post;
$args = array(
	'post_type' =>'sample',
	'numberposts' => -1,
	'orderby' => 'date',
	'order' => 'ASC'
);
$sample_posts = get_posts($args);
?>
<?php if($sample_posts) { ?>
<?php
$i=0; //start post count at "0"
foreach($sample_posts as $post) : setup_postdata($post);
$i++; //add 1 to the total count
?>
<div class="portfolio-box  <?php if($i===4){ echo 'remove-margin'; } ?>">
    <?php if ( has_post_thumbnail() ) {  ?>
    <a href="<?php the_permalink(' ') ?>" title="<?php the_title(); ?>" class="opacity"><?php the_post_thumbnail('portfolio'); ?></a>
    <a href="<?php the_permalink(' ') ?>"><?php the_title(); ?></a>
    <?php } ?>
</div>
<!-- END home-approval-box -->
<?php
//reset the count to "0" and clear the divs
if($i===4){ echo '<div class="clear"></div>'; $i=0; } ?>
<?php endforeach; ?>
<?php } wp_reset_postdata(); ?>

I would now like to re-use that string, but for certain child pages. What would I need to adjust to make it work?

Thanks