I have been volunteered to teach this ping in my son's Den. I will admit to having a similar reaction to most here when I first heard of it. (My initial reaction to "explain why it is important to have a rating system" was "Well, you have to know who you are better than !" Whoops.)
I have decided however to concentrate on the more technical aspects. We are talking about 8-9 year olds here, so I'm going to start with how to install a system, including what the different jacks are, the difference between composite, component, HDMI, etc. I'm also going to cover some history of video games, Space War, Pong, Atari 2600, show some classic gaming systems. Also cover online safety, and exchanging Nintendo friend codes (which I've found most of the other kids and parents actually don't know how to do).
Then top it off with some time in the basement arcade:
http://www.westnet.com/~chris/arcade/MyBasement/