Some brainstorming....
Each kid maintain a self-rated chart on skills and set their goals for skill improvement. Match resources to skill areas. Get some online resources for them. Have you used AMC's math club site for ideas?
Since you seem to have three different groups, define an entry scoring level to each group. Basically create a progression of attainable aspirations. Have mentoring between groups. Sometimes explaining a method will help the more advanced person formalize their own thinking and processes.
Other than whatever AMC they had in the late 70s, I lobbied (unsuccessfully) for 4 or 5 years to get a math club when I was kid. Definitely, kudos for making it happen.
