Why don't you include great children's literature?
Because the list is for adults.
Usually people ask this question if they are plumping for adding a Doctor Seuss story or some other remembered favourite from their childhood. Some stories written to entertain the young go on to become classics for all ages. The Alice in Wonderland books, for example, or Grimm's Fairy Tales. The Lion, the Witch and the Wardrobe and Anne of Green Gables are twentieth-century examples, though we'll see how long into the twenty-first century they last. Tolkien's hobbit epics may be considered works for adolescent and adult readers alike.
Sometimes adults enjoy reading certain books, like Seuss's Cat in the Hat, to kids and feel the books should therefore be considered as important as weightier adult fiction. But my criterion for the list has been whether the work is read by adults as adults for their own enjoyment and edification. Reading to entertain kids is not enough.
Right away some reader is going to protest, "But I like to read Green Eggs and Ham for myself!" Yes, and I get a kick out of going back and looking through the Hardy Boys and the Enid Blyton adventure stories I devoured as a youth. But that's nostalgia—a delightful wallowing in a since-discarded naïve worldview. It may be fun and it may even be healthy to regress in this fashion now and then. But it's not reading as an adult. It's pretending to be a child.
The children's books that eventually make it onto the adult list, like Lewis Carroll's works, usually offer a level that goes over the head of kids. Another factor to consider is the ongoing impact the work has. Alice’s Adventures in Wonderland and Through the Looking-Glass are embedded in our culture, almost as much as such adult standards as Hamlet and the Bible. They are continually referenced by serious writers, films and television. The situations and expressions are part of our everyday conversation. An adult has to have some familiarity with Alice to understand much of what's being said and written around him. When Green Eggs and Ham reaches that status, I'll add it to the list.
As always, there are some tough judgment calls. I've included Winnie-the-Pooh as approaching this status, but I have to admit I wonder about that decision.
I've kept out The Secret Garden, another one I've agonized over, consulting other sources as I have with all works. In the end, I've determined to my own satisfaction that this story has faded. It's got a fussy, old quality that isn't wearing well—at least not with new readers—laying its chin-up sentimentality too bare, with little of enduring interest. You can argue this (please do!), but my judgment of the general consensus now is that it's not one for the ages.