I don't get why anyone things they did something wrong. I'm a huge animal right supporter (and a vegan) so I obviously don't like dead animals, but If I were in charge, I would've shot the gorilla even sooner. He was throwing the kid around the moat, and clearly acting aggressive, at least from the clips I saw. A 500 pound gorilla would take 10-15 minutes to tranquilize--it's hard to tranquilize even small animals. And now you have a 500 pound gorilla with a child, whose just been shot with a sharp piece of medal. Chances are, he's gonna be PISSED, and you'll have seriously endangered the life of a child.
The parents shouldn't be charged, don't be ridiculous. Unless they literally dropped the kid off at the zoo and headed down to the Barely Legal Babes Club I don't imagine that they in any way allowed this to happen. They might have stepped out to the restroom, or been turned away, looking from a friend. Or maybe they let him climb up a bit on the fence, to get a better view. Kids can move fast, and many zoo fences aren't very high.
It's hilarious that the same people who say "KIDS ARE TOO SHELTERED... PARENTS GET IN TROUBLE FOR DOING ANYTHING THESE DAYS!" are the same people who say "CHARGE THE PARENTS WITH A CRIME BECAUSE THEIR KID DID SOMETHING A KID WOULD DO!"