Me and that Grace sit together on the school bus. Except for not today we didn't. Because today Grampa Miller drove me.
(Junie B.Jones)
In 2010, when I asked the meaning of the highlighted sentence in a Korean forum, a man replied it’s a wrong sentence. And I got a new one yesterday that says it’s a double negative, and meaning “Except for today we did.” Which one is right? If the second one is right, would you parse the original sentence for better understanding?