I think Junior First Lego League might cast a wider net. http://www.usfirst.org/roboticsprograms/jr.fll

Also, I'd make sure that some math-oriented websites are available for kids who want to explore and practice in different areas at their own pace.

I've been brainstorming ways to have a math club focused on fun rather than competitions, done right it might bring in more kids as even kids who are great at math are not necessarily interested in just doing math or competitions without a nudge. My DS8 is nuts about math, but other kids in his GT class just see it as another school subject.

Some of these ideas: Fun math and science videos, game programming (or just Scratch), a kid ran math and science video show, a business simulation club.