For little kids, How the Grinch Stole Christmas. Not because it’s about Christmas, but the structure of the story - where the villain gets away with everything, but still fails to get what he wants, teaches a great lesson about human nature.
For kids who can read chapter books, Stuart Little. Life itself is open-ended, like the book.
Encyclopedia Brown and Ramona, I’m not sure they were life defining but I really liked them as a child and I hope my own does too…just as soon as I can get her to stop eating the books.
Not a book, but as early as I can remember I identified with and rooted for Daffy Duck, not sure if the character changed me or the foil inherently appealed to me.
The question isn’t what book or short story you want your kids to read first.
Anne of Green Gables. My mother bought her the box set when I was pregnant, and it was one of my favorite baby gifts. I LOVED those books when I was a kid.
I’ve been thinking about this, and I can’t really answer. The way I see it, if my kids know just one book or story, that’s a sign that I’m a failure as a parent. I’d much rather my children read books I’ve never heard of, but many of them, than any single book that I love.
Chronos, I don’t mean that you’re choosing for your child to know only the book/story you choose. Rather, I mean that you want to be sure the child knows the work in question.
Answering my own question: given the immersion of theism and superstion of our culture, I would really want any child of mine to have read Demon-Haunted World before she was 16.
Swallows and Amazons and the rest of the series.
I have hardback copies all ready for bedtime reading.
I never read Penrod to my kids, but I read it myself when I was a kid. Reading books from times long ago is really great in that it lets kids know that things weren’t always the way they are today. They give a good sense of history. Look how the family dynamics evolved over the course of the Ramona books. We read lots of Nancy Drews from the '50s, but also a few of the reissues from the '30s, and the differences were very interesting and instructive.
Moderator Note
Guizot,
While it may be a pretty simple book, “One Fish Two Fish Red Fish Blue Fish” is not yet in the public domain and is a work protected by copyright. Quoting the book in its entirety is a violation of copyright well beyond the bounds of fair use and not permissible on the SDMB.
No warning, as it was of course an honest mistake. Carry on.
Thanks!
RickJay
Moderator
Why thank you.
At the risk of sounding like W Bush, probably some part of the Bible. Outside of that, “Stranger’s Eyes,” by Dmitri Bilenkin.
Honorable mention to “By the Waters of Babylon,” by Stephen Vincent Benet.
Adventures of Huckleberry Finn
The childrens’ book Zen Shorts was one I didn’t read as a kid, but I did read it in a bookstore as an adult while my wife was pregnant, and bought it on the spot in hopes my kid would love it as much as I did.
when she is old enough to start asking moral questions i plan to have her read “Ender’s Game” by orson scott card. i didn’t read it or any of the series until i was an adult but they were still just as relatable.
Not a parent, but I have a few that I want my kids to read:
Harry Potter, naturally. Of course, it won’t be the same now that no kid of mine will experience the books for the first time, but they’ll still be fun reads.
Calvin and Hobbes taught me a LOT of vocabulary.
Watership Down is just an incredible book.
The Hobbit when they’re youngsters, and LOTR as they get older. LOTR was a bit dense for me as a kid, but I enjoyed them as a teen and The Hobbit is always good.
To Kill a Mockingbird, because it’s such a powerful story. I actually saw the movie first as a kid, and really liked it, but the book would be good too.
Roald Dahl’s stuff, like Charlie and the Chocolate Factory. Because they are childlike but not TOO childlike.
The Chronicles of Narnia, for the fun fantasy elements, not necessarily the Christian stuff.
Crap, I just remembered that you said “one,” not “a list.” Oh well.
The Wind In the Willows – I remember a kind of melancholy that moved me in a subtle yet profound way, and a vivid evocation of place, when I read it at age 8 or so.
At age 6, a good preparation for this sort of book would be the wonderful Frog and Toad are Friends and* Frog and Toad Together*.
Oh yes, I think this is a really good one. It’s one of the only books that drew a genuine emotional reaction from me.
Just keep her away from Card’s politics and religion and you’re golden. 
To Kill A Mockingbird. Examples of how people of integrity should live. Role models for boys, girls, friends, fathers and family.
It’s a creepy choice, but Shirley Jackson’s The Lottery really moved me as a 4th grader. The random quality of a mob picking one person to ruin, and the way Jackson described that awful fear of being chosen for no real reason, I can still feel that all these years later.
I wasn’t ever the bullying type, but if I had been this would have cured me, and it certainly made me stand up for the underdog whenever I could.
Or 
The Phantom Tollbooth because it just rocks.