Any dog can become vicious without proper handling and care. I've met some super sweet Pits, Rottweilers, and Dobermans. I've also met some mean as heck Pomeranians, Dachshunds, and Labs. Seriously, its all in the way they are cared for. Sadly, some people are terrible pet owners who either ignore their pets or purposely make them vicious. When aggressive dogs get out and start to form a pack its really bad.
I'm sorry that your grandchild was injured by a Pit, but that one Pit and its owner are to blame, not the entire breed. Its kind of like saying all Germans are stone cold Nazis...almost 70 years later.