[Wordpress] Display Child Pages on the Parent Page (including the Parent Page itself)

wp-page-subpage-menu.php

<?php
/**
 * Display Child Pages on the Parent Page
 * (including the Parent Page itself)
 *
 * Sources:
 * - http://codex.wordpress.org/Function_Reference/wp_list_pages
 * - http://www.wpbeginner.com/wp-tutorials/how-to-display-a-list-of-child-pages-for-a-parent-page-in-wordpress/
 * - http://www.svennerberg.com/2009/02/creating-a-submenu-in-wordpress/
 */
function my_list_child_pages() {

	if (is_page()) {

		global $post;

		$args = 'echo=0&sort_column=menu_order&title_li=';

		// Check whether page is parent/child page
		$page = ($post->post_parent) ? $post->post_parent : $post->ID;

		$parent   = wp_list_pages( $args.'&include='.$page );
		$children = wp_list_pages( $args.'&child_of='.$page );

		// If the current page has/is child page
		$list = ($children) ? '<ul>'.$parent.$children.'</ul>' : '';

	}

	echo $list;

}

Hi, I try to use your script but since I'm new in the world of WP, I don't know how to use it.
Could you guide me please?