class pages_from_nav extends Walker_Nav_Menu
                            {
                                function start_el(&$output, $item, $depth, $args)
                                {
                                    $parentPageID = get_the_ID();
                                     global $wp_query;
                                     if ($item->post_parent == $parentPageID) {
                                      $item_output .= $item->object_id;
                                      $item_output .= ",";  
                                     }


                                      $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
                                }
                            }

                            function jawonio_pages_from_nav() {
                                wp_nav_menu(array(
                                    'menu'            => 'Main Navigation', 
                                    'container'       => '', 
                                    'menu_id'         => 'site-nav',
                                    'depth'           => 0,
                                    'walker'          => new pages_from_nav()
                                    ));
                            }