List all child pages of a parent page in WordPress.

wp-list-pages.php

<?php
  global $post;
  $ancestors = $post -> ancestors ?? '';

  if( isset($ancestors) ):
      $ancestors = array_reverse($ancestors);
    if( isset($ancestors[0]) ):
      $current_toplevel_menu_id = $ancestors[0];
    else:
    $current_toplevel_menu_id = $post -> ID;
  endif;

  $args = array(
    'child_of'        => $current_toplevel_menu_id,
    'post_type'       => $post -> post_type,
    'title_li'        => '',
  );

  wp_list_pages($args);
  endif;
?>