How to echo excerpts with wp_list_pages?

If you want to make use of all the nifty filters for the title and excerpt/content (and why would you not want that?) you should loop through a custom query instead of using get_pages and the pages’ plain contents:

<?php
$args = array(
    'post_type' => 'page',
    'post_status' => 'publish',
    'posts_per_page' => -1,
    'post_parent' => $post->ID,
);
$query = new WP_Query($args);
while ($query->have_posts()) {
    $query->the_post();
    ?>
    <h2><a href="https://wordpress.stackexchange.com/questions/136190/<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
    <?php
    the_excerpt();
}
wp_reset_postdata();
?>