Best to stay in the shade during the hours when the sun is at its hottest: from 11AM to 4PM.
If you do venture out into the sun, wear enough clothes. Children should wear a bathing T-shirt when they go swimming as well as a sunhat.
This also goes for adults. Sunglasses are advisable in strong sunlight too.
Use sufficient sun cream too on exposed skin. Combine all three measures as much as possible to provide sufficient protection to the sun's rays.