How do I pick the best sunscreen for a child?

UVA & UVB protection. Check out the environmental working group webite, they list the safest sunscreens, but at the top of this list are: adorable baby, aveeno (oatmeal) baby natural, babyganics and badger.
Look for zinc oxide. The best sunscreens are zinc oxide based. Zinc oxide reflects the sun and is not abosrbed into the child's skin.