If you think 2 and 3 year olds are too little for a science lesson, I am about to prove you wrong! You can use the following project to talk about the varying degrees of solubility of different materials.
Here's what you do: First, I mixed food coloring and water in the different compartments of an ice cube tray. Then, I laid several coffee filters on wax paper. The kids used a pipette (or eye dropper) to drip the colored water onto the coffee filters. They were fascinated with the way the colors bled an blended on the filter. We noted how the colored water just beaded up on the wax paper. To finish, I folded the dried filters into butterfly wings and added a pipe cleaner to hold the wings together and to form the body.