Get wordpress page children ID's

page_children.php

 <?php
 // Setup $post
 glopal $post;
 // Store current post ID
 $current_post_id = $post->ID;
 
 
 // Note the slash here refers to global namespace.
 $children = new \WP_Query(
            array(
                'post_type' => 'page',
                'post_parent' => $current_post_id,
                'fields' => 'ids'
            )
        );
array_unshift($children->posts,"{$current_post_id}");
$children_id_arr = $children->posts;