I think you're just worrying too much.
It's an optional club, and you're not excluding kids who aren't "good enough" at math, right? As long as anyone can join, you're providing more opportunities, not taking anything away from anyone.
I would counsel you to approach it from the stance that it's a fun group for kids who enjoy math, though, NOT that this is in some way to boost the inadequate curriculum. That's your motivation for doing it, sure, but no one needs to know that!
Make it all about the fun and the social, not AT ALL about the curriculum, and I think you'll be fine.
