If possible, meet somewhere other than in a class room. The students will be in your math group for fun, but a class room carries reminders of tension and work.
You may be able to motivate your students by including applied mathematics. Where possible, go over the math in the meeting room, and then take the math out of the school for the actual applications. If possible, have field trips. At the end of each semester, hold a mini-conference, complete with formal presentations and published conference proceedings, and invite local dignitaries (officials of the school and the school system, and officials of organizations where you hold the field trips). Parents may be able to help to organize the mini-conferences. If possible in your area, seek parents who are in scientific fields, and parents who have experience in organizing conferences.
Math applications are limited in middle school compared with high school, but your students can handle some types of problems graphically rather than numerically. You can emphasize that as the students continue in math, it becomes not only more powerful but easier.
Rather than call the organization a "club", consider a name more fitting for a serious academic field; your students may want to call it something like "The <school name> Applied Mathematics Society".