I've done quite a few through the years. The main thing I try to leave them with is the idea that mapmaking is "intelligent tracing," always working from a source (like an aerial photo) with more detail than you'll actually use, and that your job is deciding what to label and what to leave out. I generally try to persuade the teacher to do an exercise where they use tracing paper and an aerial photo of the block where the school is. Or I've had them make a map (floor plan) of their classroom by counting floortile squares. But the point is for them to decide what features get shown.
If you've done any sort of field-checking, that's usually pretty interesting to kids. (I had in one PowerPoint a picture I took of a sign in rural Wyoming saying "No Trespassing; You Will Be Shot At" and that's all they could focus on.) Drawing a couple of buildings in OpenStreetMap and a few minutes later having them show up on the website is also an impressive demo nowadays.
I also work a bit on the concept of symbolization, and how that changes from scale to scale (runways and taxiways with planimetric detail-->just the runways-->a mere airport symbol-->airport not shown at all). I used to talk about symbols that can't actually be seen on the ground, such as contour lines, but that might be a remnant of the past and is a bit much for third-graders.
Another thing I've sometimes gotten into with older kids is how we could make maps before airplanes and satellites, and I'll show how a 19th century map of the Iberian Peninsula compares quite favorably with a satellite image. How could we do that?? I just talk about how a plumb bob on a protractor at noon tells you your latitude, and how if you set an accurate clock when you sailed from London, you must be a quarter of the way around the world, or 90º west longitude, when it reads 6 pm at local noon. Put enough lat-longs together and you have a coastline map.