Create a craft that will go under the grassy cotton ball. For example, Delaware State University recommends having kids decorate a raw potato to look like a head using markers, craft eyes and card stock and growing grass out of the top to look like hair. Slice off the bottom so the potato stands straight and scoop out a small hole on top for the cotton ball to sit. Alternatively, decorate a small plastic tube, such as one that holds an individual serving of stacked potato chips.Turn the tube upside down and set the cotton balls on top of the concave bottom.
Moisten the cotton balls with water. Set a moist cotton ball on top of each craft. For the potato, push it into the scooped-out hole. For the tube, two or three cotton balls may look better than just one.
Sprinkle a pinch of grass seeds on top of the moist cotton balls and set the craft next to a sunny window.
Fill a spray bottle with water and mist the cotton balls and seeds every one to two days as necessary to keep the cotton balls moist. Let preschoolers watch the grass grow on their crafts.