Use WP_Query
class to do it:
$the_query = new WP_Query( array(
'post_parent' => $post->ID,
'post_type' => 'page',
'posts_per_page' => 1,
) );
// The Loop
while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<h2 class="subpagetitle">
<a href="https://wordpress.stackexchange.com/questions/55118/<?php the_permalink(); ?>" rel="bookmark" title="<?php the_title(); ?>">
<?php the_title(); ?>
</a>
</h2>
<?php endwhile;
// Reset Post Data
wp_reset_postdata();