I have kids doing pushups, free-squats (no weight, just squatting down), jumps and sprints at age 8.
I wouldn't introduce any weight-bearing exercises till puberty.
There's nothing wrong with starting any kid, at any age, to eat right though. This is the one thing not taught to anyone by their parents, teachers, the school system, etc.
I also agree with everyone else, that when working with younger kids, keeping it "fun" and having you providing positive encouragement, are what will make or break them. The more time they have touching the weights/bars and doing the lifts the better - a little less talk and a lot more action...
When dealing with kids between the ages of 12-14, they need a lot of public and open encouragement. A good example is if you see a kid doing something right (even something simplistic), stop the group and have them watch the kid do the exercise. Explain to the group while the kid is doing the exercise, the key points that are making this kids' movement correct. Explain the benefits, etc. The kid that is doing the exercise is going to feel fantastic and the others are going to do their best to get the same kind of public encouragement. This is the kind of stuff that keeps this age group coming back for more and giving you their very best.