public function children(CategoryHandler $handler, $categoryId)
 {
     $parentId = Input::get('id');
     if ($parentId === null) {
         $category = $handler->get($categoryId);
         $children = $handler->progenitors($category);
     } else {
         if (!($parent = $handler->getItem($parentId))) {
             throw new InvalidArgumentHttpException();
         }
         $children = $handler->children($parent);
     }
     return Presenter::makeApi($children);
 }