You might suggest that their first effort use the same pattern as that used by Felice Frankel, with a substance in the center that will be attracted to magnets arranged in a surrounding circle. The stronger the magnets, the better your results will be. Experiment with placing the paper on top of or below the magnets. Try dropping several small items containing iron simultaneously or just one at a time, letting each item move towards the magnet nearest where it falls, trailing paint or food coloring behind it. Encourage participants to be as daring and creative as possible.
Persistence is the engine of good art, as well as good science.