A good sunscreen should block UVA and UVB rays of the sun, and should not break down in the sun so that it continues to do its job. It should be safe for children and adults. But the Environmental Working Group (EWG) has found no sunscreen that will do all this.
Be aware that sunscreens cannot over complete protection from harmful UVA and UVB rays of the sun. EWG recommends limiting exposure to the sun and dressing to protect your skin from its aging effects and cancer.
"While the Food and Drug Administration (FDA) has two systems that theoretically regulate sunscreen safety and effectiveness, one has never been implemented and the other is optional and rarely used. The upshot is that there are no mandatory requirements about what sunscreen manufacturers can (and can’t) put into their products, and what kind of claims they can print on the label. The Connecticut Attorney General has called the current situation a 'marketing Wild West' in which 'sunscreen makers can make claims that are unproven and untrue.' "
http://www.ewg.org/2010sunscreen/faqs-2010