<?php if (has_children()): ?>
 <section class="full-width subnav">
  <div class="container">

    <nav class="row text-center">
      <div class="col-12">
        <?php
          $args = array(
            'parent' => $post->ID,
            'post_type' => 'page',
            'post_status' => 'publish'
          );
          $pages = get_pages($args);  ?>
          <ul class="list-unstyled">
          <?php foreach( $pages as $page ) { ?>
           <li>
              <a href="<?php echo  get_permalink($page->ID); ?>" rel="bookmark"><?php echo $page->post_title; ?>
              </a>
          </li>
          <?php } ?>
        </ul>
      </div>
    </nav>

  </div>
</section>
<?php endif; ?>