80% of my Asian friends took this extra math class as kids: any other ideas?

Kumon is not too bad. (Disclaimer: my sister-in-law in Taiwan used to be a Kumon teacher there, back before when I had children.) Some of the stronger students in the supplemental math program I teach are Kumon alumni.

But there are plenty of other good math programs. I use ALEKS regularly for all of my children,

http://www.aleks.com/

there is also EPGY,

http://epgy.stanford.edu/

and especially there is Art of Problem Solving,

http://www.artofproblemsolving.com/index.php?

with its vibrant online community and very challenging online courses.

And of course Khan Academy

http://www.khanacademy.org/

is free to users (supported by foundations) and still developing. And there are local math circle programs

http://www.mathcircles.org/

and supplemental class programs like mine.

So Kumon is good, and perhaps helpful for your family, but it is not strictly necessary.

Presumably Khan Academy is a major alternative these days...

Aops is the best