Keeping all the kids engaged is easiest when we have one project that everyone can work on. Especially if it is something they can get used to (like thinning and transplanting lettuce), and can then see when others are having trouble and can offer help.
Based on this idea, before the 7 year-olds arrived on Wednesday I made a quick decision to build a little mini garden within the garden. All I did before the group arrived was stake out a little 6'x6' plot in a sunny part of the garden next to the peppers and the herb spiral. When the group walked in, we set to weeding and tilling the little plot. The group (about 20 kids) outsized their little garden somewhat, but the kids were generally pretty good about sharing tools.
Then we mounded five rows, and I measured out 6" across each bed where they would be planting onions.
There is a kind of balance between working on a task and experiencing the garden, the trouble is, because there are only two garden counselors (Olivia and I), usually only a few kids get totally immersed in the activity, and once they do they are full of so many questions that they take up our attention. I find I am more apt to focus my energy on these kids, because they will continue on their own when they feel confident, whereas the other kids who tend to hang around the path with the other counselors and talk need both encouragement and direction. Up until now I think we have done a good job though, once the motivated kids are involved in something we can shift our attention to the other kids.
What it comes down to is really just getting the other counselors involved. Right now it's pretty obvious that some counselors don't really feel comfortable doing much more than helping the campers stay on task, or just sit on the sidelines also. While we spend the first few minutes of each session asking campers about their experiences gardening, the counselors usually linger around the fringes. I think in the future we'll do more to orient the counselors to the garden so everyone can feel confident to make choices.