The following project should be able to be done by 4th-graders with supervision.
Relative Power consumption of 'standard' (resistance wire) lights and LEDs.
Using a simple set of flashlight lamps - one a filament type and the other an LED - you can demonstrate the power savings of using LEDs. With a fixed-voltage power supply (batteries) and an ammeter connected to the circuit, you can show how much current flow is used in each case. Using the equation for power you can calculate the difference in power (watts) used to operate each lamp.
This difference can be related to the amount of fossil fuels required to generate the power needed, and thus the savings in CO2 emissions gained by using the LED instead of the filament-type lamp.
You can also find some very interesting science "Infographics" on Pinterest for a variety of disciplines. There are very many specialized modifications of the Periodic Table that may be used to spark curiosity and learning in elementary school children. https://www.pinterest.com/acschemclubs/great-chemistry-infographics/