Get 3 white shelled hard boiled eggs. Coat one egg's shell with fluoride toothpaste. Put one egg in a cup of water, one in Coke, and the one with toothpaste coating in another cup of Coke. Leave for a day or two. The Coke egg will turn brown but the toothpaste coated one will be protected.
Talk with your child daily about the importance of a healthy smile and brushing their teeth!