Display children's order numbers
If you're getting all the descendants in the for loop, you can use loop to get the index of the current iteration:

{% for chapter in entry.descendants %}
    &lt;div class="part"&gt;&lt;span&gt;Part {{ loop.index | numbersToWords }}&lt;/span&gt;&lt;/div&gt;
{% endfor %}


See the documentation for loop in the Twig docs for more details.

This gives you the index of the current iteration in the for loop though, not the actual position of the child in the structure. So if you query only a subset of the descendants, or reorder and manipulate the descendants, this won't work.