The main consideration with infant sunscreens are they should be Zinc Oxide or Titanium Dioxide based only; not only will these be much less irritating1, but they are considered safer. Infants absorb a lot more through their skin than older children and adults, and so the Oxybenzone and other organic chemicals in many adult (and even some child) sunscreens are absorbed through the skin much more than they are in adults2.
Some groups3 consider oxybenzone dangerous as an enocrine disruptor and potential cancer risk (while not identical, this is the same general type of issue as with BPA). That is disputed by other groups, including many doctors, and needs additional testing and research; but for now, at least for us, we use zinc oxide only sunscreens as they're less bothersome for our skin and they're quite good, so we see no reason to take our chances. (We also use them liberally, and don't care if they're a bit greasy or make our skin white, the main concerns with Zinc Oxide based sunscreens.)
1The Mayo Clinic guide to baby sunscreens
2Medscape - How Does Infant Skin Differ From Adult Skin (Lio, 2011)
3The Environmental Working Group: The Trouble with Sunscreen