查看的代码wp_update_nav_menu_item() 第三个参数是关于您尝试更新/创建的菜单项的数据数组。该数组中接受的项目之一是menu-item-parent-id. 如果将父菜单项的ID值作为菜单项数据数组的一部分传递给该键，则问题应该得到解决。

编辑：我还没有对此进行测试，但从理论上讲，假设您之前所拥有的一切都是正确的，这将适用于您

add_action(\'after_switch_theme\', \'my_after_switch_theme\');
add_action(\'after_setup_theme\', \'my_after_setup_theme\');

function my_after_setup_theme()
{
    register_nav_menus(array(
                           \'primary\' => __(\'Main Menu\', \'\'),
                       ));
}

function my_after_switch_theme()
{

    $menu_check = get_option(\'menu_check\');
    if (!$menu_check) {
        $primary_menu_items = array(
            \'Home\'        => array(\'url\' => \'/\'),
            \'Menu item 2\' => array(
                \'url\' => \'/\',
                \'sub-items\' => array(
                    \'Sub item 1\' => array(\'url\' => \'/\'),
                    \'Sub item 2\' => array(\'url\' => \'/\')
                )),
            \'Menu item 3\' => array(\'url\' => \'/\'),
        );
        generate_nav_menu(\'Primary Menu\', $primary_menu_items, \'primary\');

    }
}

function generate_nav_menu_item($term_id, $title, $data, $parent = false)
{
    if (!$data[\'url\']) {
        return;
    }

    $args = array(
        \'menu-item-title\'  => sprintf(__(\'%s\', \'text_domain\'), $title),
        \'menu-item-url\'    => home_url(\'/\' . $data[\'url\']),
        \'menu-item-status\' => \'publish\'
    );
    if ($parent) {
        $args[\'menu-item-parent-id\'] = $parent;
    }

    $item_id = wp_update_nav_menu_item($term_id, 0, $args );
    if (!empty($data[\'sub-items\']) && !is_wp_error($item_id)) {
        foreach ($data[\'sub-items\'] as $sub_item_title => $item) {
            generate_nav_menu_item($term_id, $sub_item_title, $item, $item_id);
        }
    }
}

function generate_nav_menu($menu_name, $menu_items_array, $location_target)
{
    $menu_primary = $menu_name;
    wp_create_nav_menu($menu_primary);
    $menu_primary_obj = get_term_by(\'name\', $menu_primary, \'nav_menu\');
    foreach ($menu_items_array as $page_name => $page_data) {
        generate_nav_menu_item($menu_primary_obj->term_id, $page_name, $page_data);
    }

    $locations_primary_arr = get_theme_mod(\'nav_menu_locations\');
    $locations_primary_arr[$location_target] = $menu_primary_obj->term_id;
    set_theme_mod(\'nav_menu_locations\', $locations_primary_arr);

//update_option(\'menu_check\', true);

}