List pages including both parent and child pages

Question

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

Answer 1

I don't think there is a way to express that precisely in its arguments.

You could work around that by building array of page IDs which includes parent and children set as desired.

$pages = get_posts( [
    'post_parent' => $page_id,
    'post_type'   => 'page',
    'fields'      => 'ids',
] );

$pages[] = $page_id;

wp_list_pages( [
    'include' => $pages,
] );

Answer 2

Here is the code with help form @Rarst that shows children page links on both parent and child pages.

<?php 
    global $post;
    $children = get_pages( array( 'child_of' => $post->ID ) );
    $hasChild = (count( $children ) > 0 );

    $page_id = ($hasChild) ? $post->ID : wp_get_post_parent_id( $post->ID );

        wp_list_pages( array(
        'title_li'    => '',
        'child_of'    => $page_id,
    ) );
?>