kirbysubpages.php

<?php foreach($page->children()->sortBy('alphabetize', 'asc') as $subpage): ?>
  <a href="<?php echo $subpage->url(); ?>">
    <div class="worktile">
      <img src="<?php 
      if ( !$subpage->featuredimage()->empty() ) {
        $img = $subpage->featuredimage()->toFile();
        echo $img->resize(600)->url();
      } else {
        echo $subpage->images()->sortBy('sort', 'asc')->first()->resize(600)->url();
      }
      ?>">
      <div class="caption">
        <?php echo $subpage->name()->html(); ?><br>
        <em><?php echo $subpage->title()->html(); ?></em>
      </div>
    </div>
  </a>
<?php endforeach; ?>