Well, the first thing I had to do was construct a "stand" for the flashlight so that it would shine directly upwards. It kept falling down, ruining the effect! I cut out two circles (using the Pringles lid as a guide) of cardstock, glued them together so that they would be sturdy, and folded them into a semicircle shape. With the fold I cut some slits going outward in a kind of star formation so that the flashlight could poke through. Then I taped all around and underneath. Finally, I cut a few more slits around the outer circumference of my circle and bent the sides down like a hexagon to make it into a "stand." Worked quite well and I don't forsee having too difficult a time helping the kids to do this.
Next I experimented with my different sized Pringles cans. The long one worked just fine, but I wondered how the small one might work? As you can see, not well at all!
I was really puzzling about this because it seems counterintuitive that the longer tube would produce a clearer image (given how light tends to dim and spread when it comes from a longer distance). Then I did some Internet searching and sure enough, if your goal is to generate a clear, undistorted shadow, you actually want your light source to be farther away. That is why the sun makes very accurate-sized shadows of birds and things hovering overhead while a light bulb would not--because the sun is millions of miles away.
This comb example from science-notebook.com helped clear this up for me considerably. My light source (in the short can) was TOO CLOSE to the bat symbol and ended up distorting the shadows. To get a clear and exact shadow, you want your light source to be far away but channeled (a task for which a Pringles can proves very handy!).
This could even be observed when I raised the height of the flashlight in the long can. Using glue sticks I created a higher stand for my flashlight, dropped it in, and sure enough the image was a bit more distorted and blurry from the light waves hitting the batman picture and sending shadows in all directions:
So in conclusion, the kids will definitely be using the long Pringles cans on Wednesday!