Define what a polymer is in a way that they can understand. Talk to them about how molecules are very small things that make up every single thing that they can see or touch, and that there are many different kinds of molecules. Explain that polymers are long chains of molecules that form very complicated structures, and that how they act depends on how they are made up and what kinds of molecules they use: some are slippery, some are sticky and some are bouncy.
Discuss the different types of polymers. Natural polymers include substances such as starch and rubber. An example of a tough or hard polymer is a section of PVC pipe. Synthetic polymers are polymers people make, and include common substances such as nylon. There are also stretchable and flexible polymers, such as polyester and polyethylene, the most common form of plastic.
Conduct an experiment to illustrate polymers. Put a cup of water in a small bowl and then stir in some Borax powder. Pour 1/2 cup of water and 1/2 cup white glue in a large bowl and stir it. Add a couple of drops of food coloring if you want. Pour the Borax and water in the large bowl and stir the mixture until a large clump forms and draws in all the liquid. Pick up the slime ball and knead it until it becomes like putty. Explain to the kids that they just created a polymer, and that the Borax reacted with the glue.
Have the kids fill out worksheets with games and activities. The Web site, Science Spot, for example, has a worksheet geared toward kids that asks them questions about what they learned with regards to polymers and also features polymer-themed games and puzzles for the kids to enjoy. These worksheets can help reinforce all the things they learned throughout the day or week.