Look for a sunscreen that blocks both UVA (long) and UVB (short) rays, has an SPF of at least 30 and is water-resistant. An antioxidant is an essential active ingredient for all sunscreens because it protects your skin from free radicals that damage DNA and can cause cancer.
It is recommended that you buy separate sunscreens for children and adults. Sunscreens for children are formulated without ingredients that could cause skin irritation – they generally contain ingredients that are not absorbed by the skin at all. Make sure you buy new sunscreen every season.