It's in the introduction to (later printings of) Ender's Game, starting on page XIX:

For some people, however, the loathing for Ender's Game transcended mere artistic argument. I recall a letter to the editor of Isaac Asimov's Science Fiction Magazine, in which a woman who worked as a guidance counselor for gifted children reported that she had only picked up Ender's Game to read because her son had kept telling her it was a wonderful book. She read it and loathed it. Of course, I wondered what kind of guidance counselor would hold her son's tastes up to public ridicule, but the criticism that left me most flabbergasted was her assertion that my depiction of gifted children was hopelessly unrealistic. They just don't talk like that, she said. They don't think like that.

And it wasn't just her. There have been others with that criticism. Thus I began to realize that, as it is, Ender's Game disturbs some people because it challenges their assumptions about reality. In fact, the novel's very clarity may make it more challenging, simply because the story's vision of the world is so unrelentlessly plain. It was important to her, and to others, to believe that children don't actually think or speak the way the children in Ender's Game think and speak.

Yet I knew--I knew--that this was one of the truest things about Ender's Game. In fact, I realized in retrospect that this may indeed be part of the reason why it was so important to me, there on the lawn in front of the Salt Palace, to write a story in which gifted children are trained to fight in adult wars. Because never in my entire childhood did I feel like a child. I felt like a person all along--the same person that I am today. I never felt that I spoke childishly. I never felt that my emotions and desires were somehow less real than adult emotions and desires. And in writing Ender's Game, I forced the audience to experience the lives of these children from that perspective--the perspective in which their feelings and decisions are just as real and important as any adult's.

The nasty side of myself wanted to answer that guidance counselor by saying, The only reason you don't think gifted children talk this way is because they know better than to talk this way in front of you. But the truer answer is that Ender's Game asserts the personhood of children, and those who are used to thinking of children in another way--especially those whose whole career is based on that--are going to find Ender's Game a very unpleasant place to live. Children are a perpetual, self-renewing underclass, helpless to escape from the decisions of adults until they become adults themselves. And Ender's Game, seen in that context, might even be a sort of revolutionary text.

Because the book does ring true with the children who read it. The highest praise I ever received for a book of mine was when the school librarian at Farrer Junior High in Provo, Utah, told me, "You know, Ender's Game is our most-lost book."