I know everybody here is the perfect parent and has had their toddler chained to them at all times and it's the fault of the parents, etc., etc., and the gorilla should have been allowed to kill the child...As usual, it's the victim's fault.
No, sometimes a child just gets away. I have, of course, seen things like Mr. "Hey hold my beer and watch this" holding his kid over the alligators alongside the highway through Gainesville, FL, but a lot of times, a child just slips away for a moment. I had a friend whose best friend as a child told her not to stare at her mother when my friend met her...because the mother had permanent hysterical paralysis on one side of her face after seeing her toddler dragged into the lion cage and eaten on, oddly enough, a visit to the St Louis Zoo.
A gorilla does not respond to a dart or a taser immediately. Simians and all wild animals are very strong and resistant, much more so than humans. Remember the poor woman who had her face ripped off by her friend's chimp - which was strong enough to tear open the door of her car - and the police could not stop it until one of them shot it. Even the owner was calling for them to kill it. An adult simian cannot be "tranquilized." The child would have been dead long before the tranquilizers took effect.