We have had family activities where each child is assigned a particular role to perform. When the 5 year old and 8 year old are assigned the role to lead the teenagers, the supportive environment of family can help them take on these roles with helpers, and gain experience standing in front of a group to lead them in an activity.
If you do not have a large family, then find a church or other type of youth group that can provide youth leadership opportunities. If when you visit such a group you notice that the adults do all the leading and do not use the opportunties to train a youth, then this is not the place for your little leader. Move on.
Another thing to look for in groups of youth is the opportunity to stand and speak in front of their peers. If everyone has to take a turn at somepoint, the rotating nature of the assignment assures that teasing is kept at a low level or they realize that what comes around goes around. This sort of speaking may be stressful for some children, and this provides an opportunity for caring adults in their lives to help them practice away from the group, and to reinforce their belief in themselves. If they are very little when doing this, like my 5 year old, the adult can also stand by their side and whisper in their ear if they forget something. This helps them learn they are capable.
Flying small aircraft is similarly stressful for adults. We take off, turn, fly, adjust instruments, with the sure knowledge that if we really blow it, the instructor pilot will jump in and take over before disaster strikes. The instructor pilot may even take over altogether for the landing because the consequences are so severe for failure.
As children have adult ‘co-pilots’ standing by for the little mistakes, knowing they will take over if they make a huge mistake, this can help them gain confidence to eventually ‘fly solo.’