While I was a voracious reader as a child, there were some major gaps in my reading -- children's books that I only discovered or read much later, when I had children of my own. In many cases these books, sadly, just don't "work" when you get older, although there are a few I appreciated all the more:
The Phantom Tollbooth (Norton Juster). When I first read this book to my own children, my initial reaction was shock -- shock that I had never even heard of it when I was a kid, and a shock of recognition -- the author's sensibility is so similar to my own that I felt like I was reading something that I could have written myself! (In fact, I did write a similar story for my 7th grade English class, called "The Land of the Numbers." Sadly, no copies survive). Highly recommended.
The Lion, the Witch, and the Wardrobe (C.S. Lewis). I never read this growing up. My only childhood encounter with Lewis was Out of the Silent Planet, which our scoutmaster had left lying around the campsite when I was at Boy Scout camp. So of course I read it in my tent instead of participating in dangerous activities like hiking and swimming. (Summer camp wasn't a total waste!) When I got older, I made a conscious decision not to read the Narnia books, thinking that they were only meant for children. When I finally did read them to my own kids, I really didn't like them. I can't explain why, but Tolkien didn't like them either, so I am in good company.
Dandelion Wine (Ray Bradbury). I didn't read this when I was a kid, and I'm glad I didn't. This is one of the many adult books masquerading as children's literature. (See my earlier discussion). If I had read it when I was younger, I would have dismissed it as tedious and uninteresting. But since I first read it only a few years ago, I was able to appreciate what Bradbury pulled off. And speaking of adult books masquerading as children's literature, here's a book I read as a child but probably shouldn't have: Tomorrow' Children. This was an anthology of science fiction about children, but it was apparently marketed as a book for children. So I picked it up in the children's section of the library when I was about 10 years old and read it. There were some genuinely creepy stories here, including one of the most disturbing science fiction stories of all: Jerome Bixby's It's a Good Life, which totally freaked me out at the time. Bixby is a two-hit wonder, also famous for writing the parallel universe episode of the original Star Trek -- the one where Spock has a beard.
A Wrinkle in Time (Madeleine L'Engle). Despite the fact that this book was heavily marketed in my childhood, I somehow never read it. Maybe I never read it because it was heavily marketed. And despite the fact that several of my own children have read it and loved it, I still haven't read it. So I cannot comment on it, except to say that any book that begins, "It was a dark and stormy night" has to be pretty good.