www.systemath.com and also try to get a copy of Teaching the Trivium by Harvey and Laurie Bluedorn
page 369 "A ten-year old is perfectly capable of jumping right into a sixth grade math textbook, such as Saxon 65, with no previous experience with math workbooks or textbooks. Skipping Kindergarten through fifth grade in math will in no way hinder your child's success in math. You do not need to wear out your child's interest and your own patience attempting to make him understand what his brain is not yeat wired to handle. Waiting until age ten, when your child is developmentally prepared to handle mathematical concepts readily, makes instruction in arithmetic very easy. What was painfully spread over five previous years, may here be compressed painlessly into as little time as a month. We are not saying that you should keep your child away from numbers before age 10. Not at all."
This is the approach my husband and I are using with our 2 third graders, and so far we are happy with the advice we gleaned from them. We are not unschooling in the area of math, just waiting for their minds to make the change from thinking in the concrete to thinking in the abstract. This happens at different ages for different children. But many studies have shown that it is very difficult to think in the abstract before age 10. Remember potty training? When they are ready it just clicks and life is much more pleasant than pulling out your hair and getting frustrated. It also gives us more time in the school day to concentrate on grammar, reading and science, etc.