Boil an egg in water with a tea bag. The tea bag will stain the egg brown. You can then give your children a toothbrush and let them brush the egg. The stains will disappear as you brush. Explain to them that this is what brushing your teeth does Show your children how to properly brush their teeth. You can usually get a packet with a toothbrush, those red tablets, and toothpaste from local dentist. This will allow them to chew the tablets. Brush their teeth and then look in a mirror and see what they missed. Cut out a large tooth out of black construction paper and let the children brush it white with tooth brush and white paint
Get as many different kinds of toothpaste as you can. Have all of your children taste them. You can even graph which ones are the most favorite and the least favorite. Put several eggs into different liquids (cola, milk, orange juice, etc). Discuss with your children what they think will happen to the eggs in the different liquids. Leave the eggs in the liquids for a couple of days. When you take it out the kids can see what the liquids would do to their teeth. Laminate a large tooth. You can then use washable markers to represent stains on the tooth and toothbrushes to brush away the stains.