If you have the right gear, Logo can be used to control robots with a serial interface. Another alternative would be to use Lego Mindstorms. In which case, you might want to take a look at LEGO::RCX (which I've never used... I don't have an IR tower)
I think writing games and graphics has a much higher bar of entry than when I was 8. I was perfectly happy with drawing blocks on the screen in basic on an Apple ][e. It was close to the games I played at the time (hell, this was before Moon Patrol, Spy Hunter and Oregon Trail ... I think Jump Man and LodeRunner were the big games of the day ... and Pinball Construction Set)
20 minutes is not enough time to do anything really significant. Even the sandwich example may be a bit much. Drawing a square might take up 20 minutes. (I'll admit, I don't know 8 year olds -- they'd probably have a better attention span than my neighbor's 5 year old ... I hope, or you're going to be driven crazy after 4 hrs of it.)
You might want to prepare for a few different possibilities, just so if the first run doesn't go well, you have a backup plan. In some of the sessions, you might have to answer lots of questions, and not get to all of the stuff you want, while in others, you're going to to rush through too quickly, and need something else to occupy time.