Nemours Children's Health advises applying sunscreen in advance of outdoor activities, ranging from 20 or 30 minutes beforehand.  Even if a sunscreen is labeled as "all-day," it should be reapplied every two hours or however frequent according to the label's instructions. If your little one is going in and out of the pool or the beach, water-resistant sunscreen should be reapplied more frequently—especially after exiting the water.  A rule of thumb is to apply 1 ounce, the amount that would fill a shot glass all over the body.