protected function renderChildren(ItemInterface $item, array $options)
    {
        // render children with a depth - 1
        if (null !== $options['depth']) {
            $options['depth'] = $options['depth'] - 1;
        }

        if (null !== $options['matchingDepth'] && $options['matchingDepth'] > 0) {
            $options['matchingDepth'] = $options['matchingDepth'] - 1;
        }

        $html = '';
        foreach ($item->getChildren() as $child) {
            $html .= $this->renderItem($child, $options);
        }

        return $html;
    }