the children are going to conduct a duckweed growth experiment. duckweed is most suitable, since the fronds can be counted individually, and they reproduce rapidly. they would start with one duckweed in an foil baking tray, and keep a count of duckweed/day, and learn how to plot and draw graphs of duckweed growth, and experiment with different variables to investigate effects of sunlight, temperature, addition of nutrients, detergent, fish, etc.