The American Academy of Pediatrics has some good information about safe sunscreen use. They suggest keeping babies younger than 6 months out of the sun and carefully applying sunscreen to older babies. Their page can be found here.
Here is a a review of the best sunscreens for toddlers.
Personally I like Coppertone water babies.. I have not had any problems with it.
Here is a long detailed report about the chemicals in sunscreen
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