I taught knapping at a summer camp last year for kids from ages 7 to 17. One of the best things you can do is start with some drawings of the principles you are trying to get across, so the process can be visualized before anybody starts breaking rock. Use a sharpie or some chalk to mark the areas you intend to remove on the rock as you demonstrate, so they can see the proper method, then let them start on chips like El Destructo said. Make sure you have some good gloves and safety glasses for them. You know how long it took you to develop knapping skills as an adult, kids take much longer to learn because the lack strength and dexterity.
Good for you for starting them out early, it will give them a good foundation and they'll have a head start on everybody else their age.
Jim