Adding Menu & Sub Menu In WordPress Not Working

i have added the below function in my functions.php in theme folder.. and i refreshed my wp-admin page.. but the sub-menu is not coming

add_action('admin_menu', 'register_custom_menu_page');

function register_custom_menu_page()
{
   add_menu_page('View Orders', 'View Orders', '', 'orders/vieworder.php', '', plugins_url('orders/small_cart_icon_20.gif'), 100);
   add_submenu_page( 'orders/vieworder.php', 'My Custom Submenu Page', 'My Custom Submenu Page', 'manage_options', 'orders/vieworder.php' );
}