It was ME! If you work hard, you can achieve great things. Ralph
Obviously Ralph is referring to the hard work he and Ivy have put into improving their hide and seek skills since Ivy's last attempt.
Actually, they're both extremely good at becoming invisible, particularly when you're in a hurry to leave the house and are trying to make sure you're not going to accidentally shut them in a bedroom. Eventually you'll find them watching you calmly from a gloomy spot underneath something. It's this sort of thing that earns them the affectionate nickname 'little devils'.