The Chronicles of Narnia series and Phantom Tollbooth have been said but they're among my favorites that I think I'd be ok intro'ing early (though perhaps not the whole Narnia series). The Little Prince is also really nice and I'd have no hesitation at this age. I think my own parents read The Secret Garden to me around 4-5 years.
One of my earlier reading memories (though on my own) were the Sideway Stories from Wayside School and the Ramona Quimby books.