Keep it simple. A good simple routine will give them the confidence to try harder things later. A good clean routine will boost their confidence. :)

I'd emphasize conditioning.....Alot of youth middies think of themselves as offensive players. Emphasize that their role on "D" is just as critical.

Ii would also work on basics--motions and jumps. Motion drills can be fun and instructive. Teach jumps from the ground up (no pun intended). Break down the jump parts and work on each.

MHSAA cheer is awesome. I've rarely seen such precision or cleanness with other styles of cheer. (Rochester just blows me away!)

A clinic is a great idea to help build up your high school program by improving the incoming freshmen ahead of time. Are you doing this through your school and with your coach? I hope so, because there are major liability issues to consider here that will require insurance coverage and some careful planning. If you and your coach are not trained to teach tumbling, stick with non-flipping drills and conditioning. If you don't have access to mats, don't stunt or keep it thigh stand or lower. Definitely work through the progressions from spotting basics through beginner then intermediate stunts, especially if these kids aren't well-trained.

Beyond that, I would google beginner cheerleading for some ideas of other skills to teach and how to present them to novice cheerleaders.

Good luck!