Let them 'find' things. Ask them to count or identify what is in their square foot. Let them draw pictures of rocks or whatever. It keeps them busy and happy. It allows you get on with the business of rock hounding.
Teenagers are a category all by themselves.
Age dependent, they may take part in the planning process of the trip. They have good ideas, ask them. Teens are stretching their intellectual and physical limits to the max. Remember who is in charge. They may be insistent, but you have the overall responsibility for the safety of the group.