Environment Teachers' Guide
Before the kids can understand that changes in the environment happen across time, they will need to know that different places in the world have different climates, different plants, different animals. It might be a good idea to begin by showing pictures of various places in Egypt, Iraq, Greece, Italy, Africa, China, India, and so forth, and discussing what is the same as at home (the sun, clouds, rain?) and what is different (sand, ocean, big rivers, plants, animals) (Those pictures are available on this site at the environment pages for each geographic area). You could have them follow the weather in different cities over the Web for a week or so, and graph the different temperatures and whether it rained or not in different places.
Once they understand that different places have different climates, you could begin to introduce the idea of climate change over time, and what sorts of things cause climate change. It might be interesting to have a debate over it: should people have cut down all the trees to make iron? What would have happened if they had saved the trees instead? How could things have been done differently? You'll probably want to relate this to modern global warming.
If your kids are good readers, you might want to try Jared Diamond’s book, Guns, Germs and Steel, which argues that most of the reason why Eurasians dominate the world and Africans don’t is environmental – that the Eurasians had environmental advantages that were not available to the Africans. I don’t necessarily agree with all of it, but it shows how important the environment is to history.