Show page title just from the first child-page in template

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();