One of the simplest ways to start teaching kids is to make them understand the omnipresence of science. From lights, fans to locomotives.
Also, you can ask them to have a separate plant bowl with their names on it. They can be taught to grow simple plants (that can fit into a bowl and needs less water) by watering them regularly. The process not only touches open botany and science learning but also helps aesthetically decorate for a green classroom. From this process, photosynthesis (without chemical formulas) can be explained.