Understanding the concept of germs is difficult when preschoolers can't grasp the idea of something so small. To help them out and have a bit of fun, give each child some hand cream to rub all over his hands. Then have children either stick their hands in a tray of glitter or pour glitter over their hands from a jar. This activity is best done over a tarp if you are indoors. Tell children to first try just dusting their hands off, then give children a towel to clean his hands. Neither will work. Ask them to wash with soap and water. The places they wash will come clean, so you can see where germs might still be lurking, and you can demonstrate how soap and water is the best way to clean.
Perform an experiment with eggs to show children which things are good for their teeth and which are not. Give each child a hard-boiled egg and put them in cups of cola or root beer overnight. Use a couple of other eggs in cups of water with carrot sticks or apple slices. In the morning, look at the eggs and see how the ones in the soda have darkened, but the others are still clean. Show the children how to brush the eggs clean with toothbrushes, and then give each child a toothbrush and cup to keep for brushing after snack time.
Make a badge in the shape of a bar of soap with bubbles or a toothpaste tube and assign one child each day to be "Inspector Clean." This child will get to be in charge of reminding other children to wash their hands after playing outside and before eating. If you have tooth brushing time, allow this child to time other children to make sure they are brushing for long enough. Even children who are forgetful will benefit from being Inspector Clean, since it will be his or her job to focus on hygiene more intensely than on other days.
Do a craft with preschoolers that will be engaging and can make washing up more fun. Shred a few bars of soap with a cheese grater and talk about how important soap is for chasing away the germs. Then show children how soap can be pressed into little round balls if you add a dab of water to the shreds. Let each child make three little balls of soap; help them press one side flat so the balls will not roll. Use a few drops of water to stick the balls together into little snowmen the children can use to wash their hands in winter.