This may seem a bit offbeat, but I'd like to recommend a children's book, The Cat in Numberland, by Ivar Ekeland. It's a story about the Hilbert Hotel told from the point of view of a perplexed, indolent housecat, written for young children who are still learning to count.

The book has a great many virtues, not the least of which is that it's wonderfully well written. It's also very short (it's a children's bedtime book, after all). It's an easy read, yet it deals with deep mathematical ideas. I think it would lend itself to a good classroom discussion both on the nature of mathematical infinity and on the nature of mathematical exposition -- in particular, whether deep mathematical ideas are, as many people believe, inherently incomprehensible.

My real recommendation would be to invite Ekeland to visit your class to read the story (and lead the discussion) himself. Ekeland is, of course, a distinguished mathematician, but more to point here, he has a wonderful reading voice. Be sure to have the students bring their blankies and teddy bears....