Listing subpages title and content and styling the first iteration differently

you should use $wp_query to retrieve pages and posts because it’s the most efficient way and you can differentiate the loop with $current_post parameter which returns the index of the loop.

It works like this

<?php 

    $args = array (
        'posts_per_page' => -1, //Showing all the pages
        'post_type' =>  'page', //Retrieving pages not posts.
        'post_parent'   => $post->ID
        );

    $the_query = new WP_query($args);
    while($the_query->have_posts()):
        $the_query->the_post();
            if ($the_query->current_post == 0){ // For first child page only
                            echo 'FIRST POST IS: ';
                the_title();
            }
            else{   //For rest of the pages
                the_title();
            }
    endwhile;
    wp_reset_postdata();

?>

Have a look at the if structure. You can use the post index current_post like that.

You can customize the query the way you like.

Reference: WP_Query Codex