List Child Pages or Subpages of a Page in the page with [gt_childpages] shortcode

list-child-pages.php

<?php 
function gt_list_child_pages() { 
 
global $post; 
 
if ( is_page() && $post->post_parent )
 
    $childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->post_parent . '&echo=0' );
else
    $childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->ID . '&echo=0' );
 
if ( $childpages ) {
 
    $string = '<ul>' . $childpages . '</ul>';
}
 
return $string;
 
}
 
add_shortcode('gt_childpages', 'gt_list_child_pages');
?>