Edwin,
Kudo to your volunteering efforts.
Some parents and students misunderstand that Math Club is for competition and not tutoring for slow learners. My DD's old school's Math club had more below average kids than Mathy kids. The teacher said they were not really interested in doing Math. the parents signed them up as after schoool activtity so that the kids can be picked up after work.
I would divide into two groups after giving one sample test (AMC-8 or Math Counts) and who score above the benchmark will be in a select group who will be prepped for competition. The other group will get fun but not so challenging Math activities. There are online Math lessons like Khan's academy (free). The study island is fun (the kids can play games after the correct answer) but there's a fee.
Discrimination is wrong but differentiation is not. When the majority of the kids in Math club is underperforming, other Mathy kids will not join in.
Good Luck!
