When I was growing up, my mom would buy supplementary math textbooks and we would work through them in addition to our school work.

It was a great help because I didn't feel very smart, and it was very helpful to me to be able to study the info on my own as well. Eventually we stopped doing supplementary homework, once we entered junior high school. Although I guess at that point, we spent our spare time on SAT prep.

Did you guys do supplementary homework/prep growing up? What are your plans for your LO?