I second the recommendation for The Phantom Tollbooth! I'd forgotten all about it until just now, but it's a lot of fun.
Also A Wrinkle in Time and A Wind at the Door, by Madeline L'Engle are fantastic. Roald Dahl also wrote good children's books, although they are sometimes a bit...gruesome. I'm having a hard time remembering specific children's books, I didn't stay in that genre long >.< Although I remember reading a lot of Magic Treehouse stories. Oh! Wayside School is Falling Down by Louis Sachar.