I'm currently listing WordPress pages and child pages on a side widget using the following code. 

    <ul class="side-links">
	  <?php wp_list_pages( array( 

		'title_li' => '' 

	   ) );
      ?>
    </ul>
This lists all of my WordPress pages, if I add the following code

`'child_of'    => $id` 

into the array I get the children on the parent page, If I go to a specific child page the list will not be displayed. I just need the list of child pages to be displayed on all pages including parent and child pages.

If it helps I've placed the following `wp_list_pages` function on this page in the following theme. https://github.com/holger1411/understrap/blob/master/loop-templates/content-page.php