Hi,

pls. add this code to the function.php of the child theme:

add_filter('wp_nav_menu_items', 'add_login_logout_link', 10, 2);
function add_login_logout_link($items, $args) {
    ob_start();
    wp_loginout('index.php');
    $loginoutlink = ob_get_contents();
    ob_end_clean();
    $items .= '<li>'. $loginoutlink .'</li>';
    return $items;
}