Get the variables of a loop from an include file
Can't you fetch all the blocks with block.topicTeaserItems.all()

{% include 'modules/module-squareteaser/_module-squareteaser' with {
  squareTeasers: block.topicTeaserItems.all()
} %}


And then you display the headlines with item.entryTopicHeadline

&lt;div class="squareteaser__wrapper _parent-3-child"&gt;
  {% for item in squareTeasers %}
    {% include 'modules/module-squareteaser/_module-squareteaser-item' with {
      headline: item.entryTopicHeadline
    } %}
  {% endfor %}
&lt;/div&gt;


A simplified version would look like

&lt;div class="squareteaser__wrapper _parent-3-child"&gt;
    {% for item in block.topicTeaserItems.all() %}
        {{ item.entryTopicHeadline }}
    {% endfor %}
&lt;/div&gt;