Have you heard of the book "Teach Like a Champion"? The book is written by a teacher who studied effective teachers and why they were succeeding where other teachers were struggling.
Basically, you have a classroom management problem. But it CAN be dealt with. Instead of asking you to read a book, here's a link to an About article which has copious links to posts on each of the techniques:
http://specialed.about.com/od/managementstrategies/a/The-...
I'd first focus on the sections Creating a Strong Classroom Culture,Building and Maintaining High Behavioral Expectations, and Building Character and Trust. If you and your fellow scout leader do not have to be teachers to access great group/classroom management skills.
Also, you could have three 'stations' set up with one main activity and two smaller ones so that your group is broken up into smaller groups, as previously suggested. You are on the right track with having a snack available first; I'd build on that with a group 'check in' where everyone shares one good thing that has happened to them or their family since the last meeting. Build rituals for the beginning and end of the meetings, and keep things very highly structured. When I planned for groups, I always had two extra activities so that if we went over time, I wasn't stuck with a bunch of bored kids.
Also, find a book of cooperative group games, too. That will be lifesaver and easy to use if you earmark the pages you want to use in advance.
Good luck!