Keep babies out of direct sun. Some sunscreens can be absorbed through babies' skin, and should not be used. Also, babies are in grave danger of overheating. Because they are so small, they cannot cope with extremes of temperature. Drink plenty of water.
Water is superior to almost all other liquids when out in the sun.
Limit your time in the sun. Go easy at first, until you know how much sun you can handle. If you get a sunburn, it can act just like a thermal burn. That is, severe burns will blister. The pain of a sunburn does not start until 2-4 hours after the burn actually occurs.