Sunscreen should be applied 20 to 30 minutes before going outdoors to be absorbed into the skin and to increase its effectiveness. Sunscreen should be reapplied every two hours while outdoors, or more often if the child is involved in water play or perspiring.
Sunscreen should be applied only to exposed areas of skin.
School-age children may apply sunscreen to themselves with adult supervision for proper application.
Sunscreen must be applied correctly in order to be effective. An adequate amount of sunscreen for application is generally 1-2 ounces.
Reflective areas (such as water and/or sandy areas) for prolonged periods of time should be avoided.

References: info/sun-safety.htm