Elementary School Math Club from Bedtime Math (Crazy 8's)

I have a kid in kindergarten (public school) and he loves math and we read bedtime math books and do math related activities with him nightly.  He asked for more math related activities in school and even though his school is good it does not provide any math or science activities for the early elementary kids.  I read about this Bedtime Math club where I can register and ask for the kit and basically chaperon and lead a math club for about 12-15 kids as an after-school club.  I was planning to do it through the school but decided against it since I was told when I raised it with the principle that she would love me to do it but I would not be able to pick the kids participating (I can only ensure my son will get to participate) since they expect the interest to exceed the 15 spots in the club and there would be a lottery and some spots will be reserved for kids who need extra help in math or from disadvantaged backgrounds.  It of course sounds fair but as I'll be leading it myself (there are no other volunteers) and am going to take half a day off work a week to do it, I want it to be fun and good experience for my son and the kids in the K classes he likes and gets a long with and not deal with behavior issues or have to supervise kids I don't know.  Thankfully I will be able to do it through my son's aftercare center and they told me they would provide the space and I can pick the kids as long as I get parental permission of the kids that I (my son) want to include in the club (which will not be a problem since I know the parents), and I just need to pass volunteer clearance and background check, etc. (which is easy since I already have it in place from helping on field trips).   I'm really excited about it but also nervous even though I know most of the kids who are going to be doing it since they are my son's friends.  Anyone has a kid participating in a club like that or has led one as a parent?  If so, how was it?  Was it difficult to lead for someone with no teaching experience at elementary school level (I worked as TA teaching math and related fields in college but I know it is not the same)?  Any other advise I should consider before I start and finalize the details.