Use wp_logout_url() in menu creation page

Add submenu with a custom link, /wp-login?action=logout, like the image below:

Add code like below in functions.php:

function wpa_remove_menu_item( $items, $menu, $args ) {
    if ( is_admin() || ! is_user_logged_in() ) 
        return $items;
    foreach ( $items as $key => $item ) {
        if ( 'Login / Register' == $item->title ) 
            unset( $items[$key] );
        if ( 'Logout' == $item->title ) {
            $items[$key]->url = wp_logout_url();
        }
    }
    return $items;
}
add_filter( 'wp_get_nav_menu_items', 'wpa_remove_menu_item', 10, 3 );