The most basic detail to teach students is that chlorophyll is a green pigment found in plants. To do this, hand each student a piece of white construction paper, child-safe scissors, and a stencil or template of a basic leaf. Instruct the children to cut out their own leaf. Then, hand them a dropper and a small bowl of water with green food coloring. Instruct students to place a few drops on their paper leaf and watch how the color spreads throughout it. Help younger kids to prevent a mess.
Towards the beginning of the school year, just as leaves start changing, bring the class outside to look at a tree. Instruct the children to look closely at the fallen leaves versus the leaves on the tree. Ask them why the leaves on the ground look different than the ones still hanging on. Explain that the difference in color comes from changes in chlorophyll as the tree prepares for winter. This is also a good introduction into the basic of photosynthesis.
Supply students with a planting pot, soil and seeds. Instruct the children to first pack the soil and then tuck the seeds into it. Explain why the plant needs water and sun before the children can begin tending to the plant. Throughout the semester, children can monitor their plant's progress while learning about chlorophyll and photosynthesis. Optionally, you can instruct a few children to put their plants in a closet without sun to show them the difference in the end results.
Older or more advanced students can learn even more about this process. While study of molecular chemistry may formally still be a few years away, you can explain the basic molecules involved in photosynthesis. Children can make models of water, carbon dioxide and sugar using toothpicks and Styrofoam balls marked with the chemical element symbols for hydrogen, carbon and oxygen. They could also use these as props representing each step of photosynthesis.