I have been teaching similar age groups for a couple of years and I find that the trick is keeping it lively and not allowing even a minute to pass where the kids are not moving from one thing to another. We will try to mix it up a bit with games that relate to karate (RyuKyu Kempo to be specific)like dodge ball where the object is to get out of the way of the ball, not catch it thus building agility, and obstacle courses where they may jump or crawl through some things but then need to stop and execute techniques at another "obstacle". I try to get the upper ranking students involved in "teaching" as well since it seems to give the younger kids something to aspire to while giving those upper ranks a feeling of progress and pride. When there are multiple instructors around - setting up "stations" where the kids cycle through a punching, kicking, blocking, moving, kata, etc. station keeps them interested.
I also find that when the class atmosphere is "tired" which happens sometimes especially in evening classes during the summer after the kids have been swimming and out in the sun all day, that getting loud seems to get their adrenalin pumping - lots of kias - katas where you kia every move and stuff like that - kids seem to wake up when someone tells them its okay to be as loud as you can. In short - teaching the kids is just about the hardest form of Martial Arts training there is - you have to really love teaching to do it.