We decided to do caterpillars this year. Our oldest is 4 and I was sort of hesitant about it because I was doubted that he would get much out of it. But, I figured I would get them anyway if only for the sake of exposure and something fun and different. He wasn't very impressed the first time he saw them because they just looked like little black pieces of string. Within only a few days they had more than doubled their size. So, when I showed them to him again he said, "wow." We read the book The Hungry Caterpillar and then I showed him the caterpillars again. He said, "those are going to turn into butterflies" in a very excited tone.
Within another few days they turned into chrysalids and we moved them from the cup to the butterfly garden to wait for them to hatch. This is day 5 after we moved them so we have a few more days to wait but should have butterflies within 2-5 days. Can't wait!