Hippos Go Berserk! by Sandra Boynton is a fabulously silly book both parents and children can enjoy. The book is fairly non-sensical but does have some pretty cool math in it. The rhyming is fun and the pictures are really cute.
Daddy Bear and I love the fact that, while Little Bear will sit still and listen, it's witty enough to keep us focused as well.
The cool thing is you can find a copy of this book for 32 cents on Amazon. It's one I'd definitely recommend adding to your collection.