Most of the time cavities are due to a diet high in sugary foods and a lack of brushing.
Limiting sugar intake and brushing regularly, of course, can help. The longer it takes your child to chew their food, the longer the residue stays on their teeth and the greater the chances of getting cavities.
Every time someone eats, an acid reaction occurs inside their mouth as the bacteria digests the sugars. This reaction lasts approximately 20 minutes. During this time the acid environment can destroy the tooth structure, eventually leading to cavities.
Consistency of a person's saliva also makes a difference as thinner saliva breaks up and washes away food more quickly. When a person eats diets high in carbohydrates and sugars, they tend to have thicker saliva, which in turn produces more of the acid-producing bacteria that causes cavities.