Set up a display of different tech areas. When I do this I usually use 2 8' tables with cardboard tri-folds with information on them about: Lighting, Sound, Costuming, Props
and a general one on Production. I also set out several different fixtures, microphones, costumes etc
. Let them touch, see and read about tech theater.
for the 20 minute talk, I would suggest that you touch on the importance of theater tech. You can create a sort of mini-monologue where you are standing center stage
giving a talk. While talking you can have some crew members running boards making various lighting and sound cues happen (both good and bad to stress what can go right and wrong) as well as different costumes and props
mixed with lighting and sound (again both good and bad... example... do you want the evil person standing in a bright pink spotlight while talking about his/her upcoming dasterdly deeds?). This can even be made to be an activity to get some of the audience involved.
They're fifth and sixth grade. They're not really going to understand the heavy theory behind lighting and sound (unless they're advanced for their age). They'll see you do some cool stuff with lighting and sound and ooooh and ahhhh, but the majority will not understand why light bends or why blue, red and green make white (because in art class it will make mud... a tactile experience you'll not overcome in a 20 minute talk). I'd stray away from those topics.
Another idea would be to stand behind a wall of 4 x 8 flats (about four of them) and one 4 x 8' flat
in the middle made of scrim
material. I would then magically appear in front of them because I'd be standing behind the scrim flat
. I would use that as my grabber saying..."Do you want to learn more about the cool things you can do in technical theater?"
Sounds like a fun evening. Good luck with whatever you decide!