Every fall the University of Michigan College of Engineering offers a course to engineering students called The Engineering Profession (Engin. 110). This course provides overview of each of the engineering concentrations.
These lectures are also available to the public online at: http://www.engin.umich.edu/courses/engr110/
Each lecture is about an hour long and there are 12 of them.
These are very cool lectures, but maybe too advanced for middle school students and younger.