Wash the small glass jars and lids with hot soapy water. Dry them off with a clean lint-free towel.
Instruct the children to wash their hands with antibacterial soap. This will help keep everything sanitary.
Show the preschoolers the butter. Explain to the children that butter is a fat that has been removed from the milk of a cow, and that heavy cream is used to make the butter.
Fill each preschooler's jar three-fourths of the way full with heavy cream. Help the children to screw the lids tightly on the jars.
Instruct the children to shake the jars. When they shake the jars, the children are churning the butter. They will need to shake the jars until the butter forms, which may take up to 20 minutes. Assist the children when needed, or read them a story while they make the butter.
Pour each preschooler's butter and liquid inside the jars into a strainer. Place a cup under the strainer to catch the buttermilk. Rinse the butter under cool water, and then press it down into the jar with a spoon.
Add a pinch of salt to each jar of butter if you desire. Spread the butter on crackers or bread for the children to sample. Refrigerate the leftover butter.