When I present to a group, I try to find the scope of the topic - that is to say find out what they want to get out of group. Do they want basic background? Do they want a specific project? Do they want to know the possibilities? I would suggest have the material revolve around a specific project that is age appropriate, and review what is required to complete that project. Be sure to have interaction with the group, start with asking everyone their Lego background, what they like best, and what they see themselves doing with LEGO in 10-20-30 years (start with yourself). I always try to keep kids involved, to be sure they are an active part of the event. Once you have a goal and the kids involved, the rest will just start flowing. You will need to watch the time to pace your goals for that particular meeting.
IBM leadership training, personal experience teaching youth classes in R/C and fish - LEGO fan for 35 years.