The American Academy of Pediatrics recommends that all infants and children should have a daily intake of vitamin D of at least 400 IU, starting soon after birth. This recommendation is based on recent research that this level of vitamin D can be given safely in babies, strengthening their immune system and preventing diseases such as diabetes and cancer.
Vitamin D supplementation of 400IU daily should continue until the baby is taking more than 32 oz of Vitamin D fortified formula or until 1 year of age. Other dietary sources of Vitamin D (fatty fish, fortified cereals and egg yolks) may be in included in the daily intake.
For more information on vitamin D in older kids take a look healthychildren.org – Vitamin-D and Sun Exposure