The American Academy of Pediatrics offers these suggestions to prevent sunburn in babies less than six months:
Avoid sun exposure as much as possible.Via Medline Plus
Dress baby in a lightweight, long-sleeved top and long pants.
Put baby in a wide-brimmed hat to shield the face.
If you can't keep baby out of the sun, apply a small bit of sunscreen to the face and the backs of baby's hands.