I am looking for the title of a children's book.?

The book is by an English author. The only details I can remember are it takes place in an unusual land. There are two main characters, I believe a brother and sister and there may be a rabbit who is King. I think I also remember the grass was blue and the sky was green. My 5th grade teacher read it too my class over 30 years ago and she said her daughter sent the book to her from England. I know this is a long shot but if anyone has any ideas to help in my search i would greatly appreciate it.