Recently the folks at child_lit were discussing children writing to authors. Someone posted a link to the Letters About Literature program sponsored by The Center for the Book at the Library of Congress. Essentially, it's a writing contest for students in grades four through twelve. "...readers write a personal letter to an author, living or dead, from any genre--fiction or nonfiction, contemporary or classic, explaining how that author's work changed the student's way of thinking about the world or themselves."
Students may enter as individuals or through their class. Check out the guidelines and rules. The entry period for this year's contest starts tomorrow.
By the way, if you know young people who want to just write an ordinary letter to an author, encourage them to use e-mail, if the author has it. Letters sent to a publisher's office could sit there for months, while if authors check their e-mail regularly they could have it in hours. (Though they may not be able to respond for a while.)