code: 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;
      }
docs: Renders all of the children of this menu.
  
  This calls ->renderItem() on each menu item, which instructs each
  menu item to render themselves as an <li> tag (with nested ul if it
  has children).
  This method updates the depth for the children.
  
  @param ItemInterface $item
  @param array         $options The options to render the item.
  
  @return string