Get Child pages of page according to their menu order in nav menu

child-menu-pages.php

<?php
/**
 * Get Child pages of page according to their menu order in nav menu
 *
 * @return array ids of child pages order by their order in menu. FALSE on failure.
 */
function get_menu_child_pages() {
    $menu_id = 0; 
    $current_page_id = get_the_ID();
    $child_menu_list = '';
    $menu_parent_id = '';
    $all_menu_loc = get_nav_menu_locations();
    
    if ( is_array( $all_menu_loc ) && $all_menu_loc && key_exists( 'primary', $all_menu_loc ) ) {
        $menu_id = $all_menu_loc['primary'];
    }
    
    if ( $menu_id && $current_page_id ) {
        $main_nav_menu = wp_get_nav_menu_items( $menu_id, array( 'orderby' => 'menu_order' ) );
        if ( is_array( $main_nav_menu ) && !empty( $main_nav_menu ) ) {
            
            foreach ( $main_nav_menu as $nav_item ) {
                
                $nav_object = $nav_item->object;
                
                if ( isset( $nav_object ) && ( $nav_object == 'page' ) ) {
                    
                    if( $nav_item->object_id == $current_page_id  && $menu_parent_id == '' ) {
                        $menu_parent_id = $nav_item->ID;
                    }
                    
                    if( isset( $nav_item -> menu_item_parent ) && $nav_item -> menu_item_parent == $menu_parent_id ) {
                        $child_menu_list[] = $nav_item->object_id;
                    }
                }
            }
        }
    }
    
    if ( !empty( $child_menu_list ) ) {
        return $child_menu_list;
    } else {
        return false;
    }
}