You need to lay down the rules with them first, as young as they may be.
Try this & they'll soon get into the habit of it. If they're running a riot, just stand at the front, and hold up your index finger. They should clap once. Hold up 3 fingers. They should clap three times. Do this a few times - holding up 1, 2, 3, 4, 5 fingers and they should clap accordingly. When you show them zero, they don't clap and there should be silence.
Always start with a song. Even thought I have a co-teacher for my Kindergarten class that I teach once a week, I start off with the "Hello" song from Junior Naver. This is great, because it becomes routine and they will expect to sing it at the beginning of every lesson.
Use candy, stamps or stickers. At random, choose one or two kids who are exceptionally well-behaved, are being active during activities and doing their work well. Bring them to the front of the class and reward them in front of their friends.
Since it's an after school class, minus points won't work well. Rather, reward them!
With YL's, remember you become an entertainer! Teach them songs with actions like "Head, shoulders, knees & toes / Eensy Weensy Spider / Hokey Pokey / Skidamarink"
I start all my YL's with parts of the body. This can take a few lessons to cover. In the first lesson, you may only just be able to cover "eyes, nose, ears, mouth". Drill this in and then add maybe two more. I have a great activity for this that they love. All you need are crayons and paper. Let me know if you'd like it.
Otherwise, give the other suggestions a try and let me/us know how it turns out:)