Group
  Fruit
    Apple
    Mango
    Banana

protected function _getItemsData() {
        $key = $this->getLayer()->getStateKey() . '_SUBCATEGORIES';
        $data = $this->getLayer()->getAggregator()->getCacheData($key);

        if ($data === null) {
            $categoty = $this->getCategory();
            /** @var $categoty Mage_Catalog_Model_Categeory */
            $categories = $categoty->getChildrenCategories();

            $this->getLayer()->getProductCollection()
                    ->addCountToCategories($categories);

            $data = array();
            foreach ($categories as $category) {


                    $subcategories = $category->getChildrenCategories();
                    foreach ($subcategories as $subcategory) {
                        if ($subcategory->getIsActive() && $subcategory->getProductCount()) {
                            $data[] = array(
                                'label' => Mage::helper('core')->escapeHtml($subcategory->getName()),
                                'value' => $subcategory->getId(),
                                'count' => $subcategory->getProductCount(),
                                'parent' => $category->getName(),
                            );
                        }
                    }

            }
            //exit;
            $tags = $this->getLayer()->getStateTags();
            $this->getLayer()->getAggregator()->saveCacheData($data, $key, $tags);
        }
        return $data;
    }

www.demo.com/fruit.html?cat=5      //5 is Apple category id

www.demo.com/fruit.html?cat=3      //3 is Fruit category id