$site->children()

$site->children(): Kirby\Cms\Pages

<nav class="main-menu" role="navigation">
  <ul>
    <?php foreach($site->children()->listed() as $item): ?>
    <li><a<?php e($item->isOpen(), ' class="active"') ?> href="<?= $item->url() ?>"><?= html($item->title()) ?></a></li>
    <?php endforeach ?>
  </ul>
</nav>