The best way to ensure safe vision is to choose polycarbonate lenses. More durable than regular plastic, polycarbonate lenses are very lightweight and shatter-proof. They also have the best impact resistance of any lens material. Polycarbonate lenses are actually constructed out of material that is identical to what is called "bulletproof glass." They also have built-in ultraviolet (UV) protection to protect the eyes from the sun's harmful rays.
Many eye doctors choose to use only polycarbonate lenses for children's glasses. Since the lenses are shatter-proof, a child's eyes won't be damaged by shards of flying glass or plastic if the glasses are hit hard by a ball or a bat.
To keep vision clear for as long as possible, polycarbonate glasses usually come with a scratch-resistant coating. Though not scratch "proof," lenses that are treated front and back with a clear, hard coating do become more resistant to scratching.
Keep in mind, if your child is planning to play sports, make sure to choose a safety frame to hold the polycarbonate lenses. Safety sports frames are made to hold up to high impact from fast-moving balls or heavy racquets.
Note: Contact lenses are not a form of protective eyewear. Contact lens wearers require additional protection when participating in sports.
Source: The University of Michigan Kellogg Eye Center. "Preventing Eye Injuries." 26 Jun 2007.