For the boy, I'd recommend Daniel Pinkwater. Some of his stuff is more for high school kids, but all of it would be great if he is at all sophisticated. His stuff is hilarious, thought-provoking, and wonderfully subversive. A more recent recommendation would be Michael Chabon's Summerland -- a fantastic novel about friendship, family, tradition, magic and baseball. Again, it may be a bit intimidating for a 12-year-old, but if he could handle Harry Potter, this will be no problem.
For the girl, I have to recommend Cat, What Is That? by Tony Johnston (Author), Wendell Minor (Illustrator). It is a beautiful poem about just what a cat is, with passages like:
It is the Slink.
It is the Sneak
on velvet toes
stalking the Squeak.
The words are rivalled by the amazing paintings, which could carry the book all by themselves. I'll look through my daughter's bookshelf tonight for more.
