Count Level 1 Childs for Custom Nav Walker WordPress

I’m trying to make my logic like this:

calling below code inside

function start_el( &$output, $item, $depth, $args ) {
$this->sub_items = 0; // declared variable as private $sub_items = 0 above inside class 
            if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ $args->theme_location ] ) ) {
                $menu = wp_get_nav_menu_object( $locations[ $args->theme_location ] );
                $menu_items = wp_get_nav_menu_items($menu->term_id);
                foreach( $menu_items as $menu_item ) {

                        if( $menu_item->menu_item_parent == $item->ID ) {
                            $this->sub_items++; 
                        }
                }
           } 
}

I’m asuming that each time $this->sub_items returns calculated value for first level childs, and I can access it inside

function start_lvl( &$output, $depth ) {
     echo $this->sub_items;
}

But unfortunately it doesn’t output correctly 🙁