The author uses a bicycle bottle dynamo, a car radiator fan, and a funnel to make a simple, and effective wind power generator. Oh an a bike axle can be used to help the thing move around. This can be a bit more pricey than some of the other builds for kids, but its an easier build than most of the other ones.
http://www.gotwind.org/diy/bottle_generator.htm
Oh the author has allot of other nice projects here too