NIH site has games, printables, scientific information, all in kid-friendly format and design.
Planetpals offers a tremendous number and variety of resources, including activities, crafts, lesson plans and book lists. Note: contains advertising.
This EPA-hosted page includes a short (2:48) video, and well-organized tabs with basic information about climate change, evidence of climate change, instruction on the scientific method, and actions to mitigate climate change and prepare for the future, and games to enhance understanding of the issue. Appropriate for elementary students, colorful and engaging.
This site includes PBS episodes of The Greens, interactive games and a comprehensive activity guide to help students understand their own roles with regard to taking care of the planet.