When kids are first learning to brush their teeth by themselves, a lot of toothpaste tends to go down the hatch. If that toothpaste contains fluoride, food coloring, preservatives, sweeteners and other chemicals (as most commercial toothpastes do), your child is ingesting these chemicals into their bodies every day, day after day. What are the long term effects of this exposure? No one knows for certain.
To be safe, purchase one of the many brands of natural toothpaste available without potentially harmful chemicals. You can find them now in most supermarkets, as well as health food stores. The Waleda company makes some great toothpaste products. More info can be found here.