I have put my book, Modern Math for Elementary Schoolers, on the Internet. The book is based on my teaching experience at LAMC, Los Angeles Math Circle, a free Sunday school for mathematically inclined children, currently second grade through high school, run by the UCLA Math Dept. I had taught my own son and a few of his buddies using the material that later became that book from the first day of their kindergarten. You can find the book at the following URL.
The book is copy-lefted. You can use it for any non-commercial purpose.