protected function getSubPageIds($pageId)
    {
        /** @var $queryGenerator \TYPO3\CMS\Core\Database\QueryGenerator */
        $queryGenerator = GeneralUtility::makeInstance(QueryGenerator::class);

        // here we retrieve only the subpages of this page because the permission clause is not evaluated
        // on the root node.
        $permissionClause = ' 1 ' . BackendUtility::BEenableFields('pages');
        $treePageIdList = $queryGenerator->getTreeList($pageId, 20, 0, $permissionClause);
        $treePageIds = array_map('intval', explode(',', $treePageIdList));

            // the first one can be ignored because this is the page itself
        array_shift($treePageIds);

        return $treePageIds;
    }