Fill a container with potting soil or earth from your garden. Collect earthworms and place them into the container. Once they make themselves at home, bring the container to the classroom. Use a watering can to wet the ground, encouraging the earthworms to come to the surface to let the preschool children observe them.
Another option is to take the kids outside on a rainy day when earthworms have emerged from the ground. This allows the kids to observe the worms in their natural habitat.
Create a class book about earthworms. Include pages in the book that describe various attributes of the worms, such as color, movement, diet and habitat. Take photographs of the earthworms during the observation to use in the book or have the kids draw pictures.
Another option is to create page templates for individual earthworm books. Let the kids draw pictures that illustrate the words on each page of the book template. Staple the books together with a construction paper cover.
Let the preschool kids create their own earthworms using clay or Play-Doh. The live earthworm observation provides the kids with a model for their homemade worms. Show them how to roll the clay into a worm shape. Encourage them to arrange the worms the way they want them to stay, since they will become stiff after setting up. After the clay earthworms are hardened, let the kids paint them for a more realistic look.
Carry the earthworm theme to snack time with cups of dirt desserts. Enlist the help of the preschool kids in making chocolate pudding as the base of the dessert. Scoop the chocolate pudding into individual paper or plastic cups. Let the kids sprinkle crushed chocolate cookies on top of the pudding to resemble the dirt. Hand out two or three gummy worms to each child, letting them place the worms into the edible dirt.