Count subpages and display a simple navigation

navigation.html.twig

/**
 * GRAV
 *
 * @tags  Grav, Flatfile, CMS, Theme, Snippet, Navigation, Dropdown, Variable, Count, Subpages, Loop, Twig
 * @url   https://getgrav.org
 */
 
<div class="container">
 
  {% set childs = page.children.visible|length %}
  
  {% if childs > 0 %}
    <ul>
    
      {% for child in page.children.visible %}
        <li><a href="{{ child.url }}">{{ child.menu }}</a></li>
      {% endfor %}
      
    </ul>
  {% endif %}

</div>