I am not a teacher, but my daughter's 4th grade class invited me to come in and present something on "circuits" to start their unit on that subject.
I brought my small oscilloscope to the class and put it on the table facing straight up, underneath their document camera, which projected a live view of it on the big screen. This worked well so everyone could see it. I had considered some kind of complicated setup using a PC to display a signal but as it turned out in this case, there was nothing extra needed.
What I was showing was the output of a HB100 microwave motion sensor (minimal $3 doppler radar module from ebay). I waved my hand in front of it, and that made a wavy line on the scope. Then I looked up and every single kid in the class was waving both arms enthusiastically. That was a fun moment! I also asked how much they think this sensor cost- they figured hundreds or thousands of $, and were amazed that it cost less than one hot lunch at their cafeteria.
As examples of circuit boards, I showed an Arduino, which no one had heard of, and a Raspberry Pi, which many of them *had* heard of, so it is a somewhat known thing among that group of 10 year olds. My daughter piped up "yes it is a real computer, and yes you can play Minecraft on it" which got many heads nodding.