Cavities aren't as common as they used to be, thanks in part to fluoridated water and calcium-rich diets. Another way to safeguard against tooth decay is to encourage daily brushing.
Toddlers and small children should have their teeth brushed by parents at least once a day. When kids are older (say 5 or 6), they should be brushing their teeth on their own. The American Dental Association (ADA) recommends brushing at least twice a day with a soft bristle brush that has a head small enough to reach back teeth, where cavities often start. Older kids should also be shown how to properly floss their teeth.
As a final note, be sure to show your child proper brushing techniques, including how to brush his tongue. Halitosis isn't uncommon in kids, and is usually due to poor brushing habits. The ADA doesn't recommend the use of mouthwash in kids under age 12, unless prescribed by a dentist.