Who's to blame? I'd probably answer "it was an accident", but the idea of an accident seems to have fallen out of all use these days - blame must always be assigned.
So if I must pick, then the parent. Some think the zoo must be proof against all misbehaviour by visitors but I don't think that's either reasonable nor plausible without impacting both animal welfare and visitor enjoyment.
The following statement is depressing:
Mr Maynard said that although the boy was not under attack, he "certainly was at risk".
Well yes. Everybody is at risk all of the time. Risk is a constant. But how great was it? Gorillas are essentially people with weird learning disabilities. They're capable of the same emotions as humans, many of the same social relationships as humans and the difference is their level of learning ability and lack of sophisticated language (they can learn some basic language, though). They're people, albeit not people like us. Is there reason to suppose that the gorilla was going to hurt the child? From the footage I've seen, he pulls the child up in the moat and then mostly just stands over it watching it. He was shot and killed as a precaution. Which might be justifiable by the zoo, but is a terrible tragedy, nonetheless.
So I guess I'd blame the parent, though I still lean towards accident however out of fashion that term is. But I'd save a little blame for the zoo. I can see why they did it, but they could have got the keeper in there who knows the gorillas (and who know the keeper) to try and retrieve the child without killing the gorilla. It would have required some bravery because they knew that they could be in trouble for not just shooting the gorilla. But that's okay by me. We can't run the world making every decision based on blame. And I suspect that's what went through the zoo staff's mind: that they'd like to go in and try to retrieve the child, but that if the gorilla did harm the child, the zoo would be held responsible for not killing the gorilla right away. And it's a shame to kill the gorilla because you're worried about being held responsible.