The number-one dental problem among preschoolers is dental decay.
One out of 10 two- year-olds already have one or more cavities
By age three 28% of children have one or more cavities
By age five nearly 50% of children have one or more cavities
Dental Health & Hygiene for Young Children
The best way to protect your child's teeth is to teach them good dental habits. They may be an enthusiastic participant, but they won't yet have the control or concentration to brush their teeth all by themselves. You'll need to supervise and help them so they remove all the plaque. They also like to copy what you do. So brushing your teeth as they brush theirs shows them how to do it. After that is a good time for you to check and re-brush their teeth to make sure all the plaque was removed.
Your child only needs a small amount of toothpaste (a peas size). It also doesn't really matter the motion they use of brushing their teeth until they are about 6 years old. It is more important about getting them into the habit of brushing and getting all the teeth. They like to just brush the front because that is the teeth they can see. Teach them to reach every tooth inside, outside, and where they chew.
Both the American Academy of Pediatrics and the American Academy of Pediatric Dentistry recommend that all children see a pediatric dentist and establish a "dental home" by age one.