It's a good idea to keep your baby out of the sun if they're younger than six months. Younger babies can't easily cool themselves off, so they're in danger of overheating or severe sunburn. If the baby goes out in the sun, lightweight clothing and a wide-brimmed hat are absolute musts.
For your baby's sake, you should recognize the symptoms of heat exhaustion. Nausea, confusion, vomiting, headache, weakness, dizziness, fainting, fever, a racing heart, and seizures are all common telltales of this condition. Immediately cool the baby off and seek pediatric care!