What is this childrens book?

I vaguely remember the details but I do remember that it was sort of a journal of a girl on earth. One day her parents tell her to pack up everything as the earth is destroyed and then they go onto a new planet. I think the planet was covered with undiscovered creatures and was very green. But then I think that at the end it turned out to be a dream but I'm not sure Anybody know the name of this children's book?