Whew, you've got your work cut out for you
You will have fun though. I started a club at our elementary school a couple of years ago. Working with the music teacher he got a grant for 20 ukuleles for the classroom at a bulk rate. He teaches it during school and together we run the club. We found that during the week (after school) is the best time to have the meeting. On Friday's their minds are too focused on the weekend.
The first meeting we had had 40 kids in it! So, big whoops on our part. Don't have too many kids at once! I think 15 is the tops. You'll need a helper too if possible so while you are teaching another can be helping the kids place fingers etc. If you have the time and more kids want to be in the club you could have it on 2 days. Unless you have A LOT of help. I found that for these young kids it is important to have a strap (cheap thin rope or 1/4in cloth ribbon) on the ukes. It keeps them from messing around with the ukes and helps them be able to form chords. I agree as above, stickers really help for the 3 finger chords! Use those colored dots from the stationary store, color can denote which finger goes where. I always teach for GCEA Great Cats Eat Apples.
Set rules! You want the class to be fun but with the young ones they need these rules. There will be behavior issues. The music teacher and I used the 3 strikes method. You set the penalty. On the 3rd strike we notified the parent. If they didn't straighten up they would be removed from the club. I'm glad to say we never had to do that! Came close though!
As for early songs the children seem to just love "We Will Rock You" (with some performing percussion on the back of the uke), "Battle of New Orleans", "Boa Constrictor". I often take a familiar popular song they listen to now on the radio and if it is inappropriate (which many are) I put new words to it. Make sure each has a binder they keep in order and have the pages ready for them (3 hole). The parent's really need to be on board to help their kids. Have a parent meeting before the club begins. You'll need to have these parents help for the club be successful. They will really need to have a uke at home for practice for the best success.
We had to eventually split our group as some learn faster than others. We were loosing the attention of the kids who "got it" and had to develop an intermediate group. Well, obviously I have tons to share about this. If you'd like to ask any specific questions and like my advice you can p.m. me and I'll give you my email address for easier communication. You will have tons of fun watching their eyes light up and discovering those kids that just seem to naturally have a talent for music. It's very rewarding!
This is the first year I have not been involved in the club and I miss it. The ukes are still in the school and I may do the club next year. But with the music teacher having his first child and with mine graduating to middle school and my starting a uke club in town things got tight. Our school does not allow a parent to teach a club without a teacher present and without special school board consent and I just didn't have the time to deal with it this year.
GOOD LUCK!