I would like to add some things to Joe's answer, which I think covers most of the safety and cost concerns.
However, picking out sunglasses for my son was much more about his comfort with them. If he doesn't like how they feel on his face, then he will take them off (whilst complaining about the brightness)! You can meet all the other qualifications, but it won't matter if your child won't wear them.
I think we end up getting him sunglasses slightly larger than his size, in order for them to be comfortable. If the bows are too short (or the elastic too tight), then the nosepieces and rims push into his face and cause him discomfort. Also, if the bows/sides rub his temples as they're put on, that initial discomfort will cause him to resist wearing them at all. Slightly overlarge sunglasses are easy to slide onto his oft moving head without risking abrasion.
I would add that we had no success with adjustable-band sunglasses. The pressure of the strap is too uncomfortable for him. The only thing he'll wear on his face with a band is his Batman mask.
Proper wide-brimmed hats may be a good solution if you can't find suitable sunglasses, but should also be used in conjunction with them. Personally, my son is much less averse to wearing hats compared to shades. Now that he's older, he'll usually wear his sunglasses without complaint. I can't imagine that his cancer risk or eye health, from only wearing hats/sunscreen for the first two years of his life is affected in any way that I would care about.