*I* loved the picture book "The cat in Numberland" by Ivar Ekeland when my kids were little. It gets at some deep concepts in a visual, fun way. It may be out of print and hard (expensive) to buy, but check your library. Here's the description from Amazon:
At Mr. and Mrs. Hilbert's Hotel Infinity, the resident cat is puzzled. The hotel is fully booked -- the rooms are full of Numbers, both Odds and Evens -- yet guests continue to arrive. When Zero shows up, there's a massive room switching, and he stays, even though Mr. Hilbert insists he's not really a Number. Then the Letters appear and everyone still has a room, even though no Numbers have left. No matter how many guests arrive or depart, the hotel accommodates them all, and is always full. But when the Fractions arrive, demanding rooms, real chaos threatens. Can an ingenious solution be found to house them all? Based on mathematician David Hilbert's famous paradox of the Grand Hotel, "The Cat in Numberland offers a refreshingly clear explanation of infinity for readers of all ages.
