If a black spot appears on the tooth, this could be the start of a cavity. Attacks from the acid contained in food will slowly destory the tooth's enamel. Choosing a suitable toothpaste with fluorine can help protect your children's enamel. Your children need a suitable concentration of fluorine for their age. Elgydium laboratories offer a whole range depending on your child's age. This one is for children 7-12 years old: 1000ppm of fluorine.