Since legend has it that the first kite was a farmer's hat attached to a string (see Introduction), why not try to find out what every-day item will serve as a kite? Buy several rolls of kite string and tie them to paper plates, plastic bags (strict supervision needed with these), paper bags, toys and more. Go out into a strong wind and take note of what will fly.
Kites differ from country to country. Go to sites such as Kite Geography and Poole Kites to study kites from around the world. Your kids will be startled by a wide variety of designs and colors.
Encourage your children to design their own kite. They can borrow designs from the Library Think Quest site (see References), or design their own. Above all, the kids should paint their kits, draw on them, affix stickers, and whatever they'd like to make them their own. Then, when the right wind's blowing, the kids should fly their kites and discover which designs fly best.