Get associated product from super attributes

   ......


    /**
     * @param \Magento\Framework\App\Helper\Context $context
     */
    public function __construct(
        Context $context,
        \Magento\Framework\ObjectManagerInterface $objectManager
    ) {
        parent::__construct($context);
        $this->objectManager = $objectManager;
    }
......
$childProduct =  $this->_objectManager->get('Magento\ConfigurableProduct\Model\Product\Type\Configurable')->getProductByAttributes($this->getRequest()->getParam('super_attribute'),$product);