Microscopic organisms fall into two kingdoms, Prokaryotes (bacteria, which do not have a nucleus enclosed in a membrane) and Protoctists (protozoans, which do have a nucleus and other cell organelles enclosed in a membrane. Most exist as single cells or small clusters of cells. There are 1000s of species of each. Most kids will have fun collecting pond water samples (get a sample of water with decaying vegetation at the very edge of a pond), looking at it under a microscope, and finding protozoans. Common types can be identified from pictures. Type into your browser:
images for identification of common protozoans