I did a quilting club at school this year. High school age and not much experience sewing. Pick a simple pattern and have all girls make the same. You will never get them finished if you let them design their own. Too many choices are a problem both with the age group and for novices. Pick a pattern that if their seams are not 1/4 inch the pattern will still work. Have a number of helpers with this. The girls will need lots of one on one help and you will have to spend a lot of time keeping the machines working, threaded, etc so make sure the helpers know how to thread the machines you are using.
I'm sure you will have a great time and be very successful with this. As a former gs leader, I commend you for getting involved. The girls will be thrilled!