Hi ForbiddenChunk! You should use the post ID instead of the post object in the post_parent argument. Take a look at this, may helps you in the future https://gist.github.com/luetkemj/2023628
<?php
$post_object = get_field('second_subpage');
if( $post_object ):
// get the post id from the post object
$post_ID = $post_object->ID;
// override $post
$post = $post_object;
setup_postdata( $post );
?>
<!-- here -->
<?php
$args = array(
'post_parent' => $post_ID,
'post_type' => 'page',
'posts_per_page' => -1,
'order' => 'ASC',
'orderby' => 'menu_order'
);
$parent = new WP_Query( $args );
if ( $parent->have_posts() ) :
while ( $parent->have_posts() ) : $parent->the_post();
?>
<div class="page_link page_link--small count_five" style="background-image:url('<?php the_post_thumbnail_url(); ?>');">
<div class="page_link__title">
<h2><?php the_title(); ?></h2>
<?php the_field('preview'); ?>
</div>
</div>
<?php endwhile; ?>
<?php endif; wp_reset_postdata(); ?>
<!-- end -->
<?php wp_reset_postdata(); ?>
<?php endif; ?>
