At about 3 years of age, your child should have all of his or her baby teeth in place. Since statistics show that close to 50% of kids have their first cavity before their first day of school, preventive measures should be taken.
We realize every child is different, but try to make sure that bottles and sippy cups are a thing of the past beyond one year of age. Fluoride toothpaste should be used (pea-size for a 2 year old, slightly bigger for ages 3-5), and sugary snacks between meals should be given sparingly.
The American Academy of Pediatric Dentistry (AAPD) says that no more than 6 ounces of juice should be given in a day. Those sucking habits like use of a soother or thumb-sucking should be stopped before age to prevent facial development issues and bite problems (buck teeth for example).
Kids this age are likely trying to be independent, wanting to brush their own teeth and all, so let them do it but keep close supervision to ensure they do it right and stay with it.