Drupal - Loop through child elements in Twig like Element::children()

{% for key, child in element if key|first != '#' %}
  <div>{{ child }}</div>
{% endfor %}

I have created a Twig filter that returns back with the children as an ArrayIterator.

mymodule/mymodule.services.yml

services:
  mymodule.twig_extension:
    arguments: ['@renderer']
    class: Drupal\mymodule\TwigExtension\Children
    tags:
      - { name: twig.extension }

mymodule/src/TwigExtension/Children.php

<?php

namespace Drupal\mymodule\TwigExtension;


class Children extends \Twig_Extension
{

  /**
   * Generates a list of all Twig filters that this extension defines.
   */
  public function getFilters()
  {
    return [
      new \Twig_SimpleFilter('children', array($this, 'children')),
    ];
  }


  /**
   * Gets a unique identifier for this Twig extension.
   */
  public function getName()
  {
    return 'mymodule.twig_extension';
  }


  /**
   * Get the children of a field (FieldItemList)
   */
  public static function Children($variable)
  {
    if (!empty($variable['#items'])
      && $variable['#items']->count() > 0
    ) {
      return $variable['#items']->getIterator();
    }

    return null;
  }

}

in the Twig template:

{% for headline in entity.field_headline|children %}
  {{ headline.get('value').getValue() }}
{% endfor %}

Use the Twig Tweak module, which, among other wonderful features, has a "children" filter:

{% for item in content.field_name | children(true) %}
  {# loop.length, loop.revindex, loop.revindex0, and loop.last are now available #}
{% endfor %}