Some do. Because most breast milk does not contain much vitamin D, the American Academy of Pediatrics says breast-fed infants should get 400 IU daily from supplement drops. Our new Ratings of vitamin D supplements included four children's supplements.

To avoid overdose, the Food and Drug Administration says you should use the original dropper that came in the package, and not to use products in which the dropper is poorly marked or holds more than one drop.

For children 1 and older, the recommended vitamin D intake is 600 IU daily. Kids ages 9 to 18 are actively building bone, so it's critically important for them to get that, as well as their recommended calcium intake of 1,300 milligrams daily.

"Our research shows that adolescent and teen girls are particularly prone to deficiency," says Clifford Rosen, M.D., of the Maine Cancer Research Institute in Scarborough and a member of a Institute of Medicine Committee that recently looked at the evidence for vitamin D supplements. "So for them, a modest supplement may be good insurance."