Whether the middle schoolers in your life are educated at home or elsewhere, the best way to introduce the economics of Liberty is Whatever Happened to Penny Candy?
This is the book that introduced me - at age 38 - to Austrian Economics. I read the book after my homeschooling wife bought it as an economics text for my oldest son. Penny Candy shook my neoclassical background to the core -- in an evening nonetheless.
After reading the book, I searched the internet for "Austrian School of Economics" and found Mises.org.
The book is a great introduction to the free market for middleschoolers. It will also benefit students in high school and college, as well as any adult who hasn't shed the statist influences of public education.
A truly excellent book!
Buy it for your children. And buy an extra copy or two to give to your friends.