I think it is partly the parents fault.
The boy didn't just fall through a gap in a fence he scaled a small wall, climbed through bushes etc.
I'm not saying that 4yos can't get away in seconds obviously they can and it often does happen and it's not down to neglectful parenting.
However - personally in this situation - in a zoo where there are thousands of people - I would not let a 4yo out of night sight and to be honest I would be totally uncomfortable without them holding my hand. Not through fear of them falling into an enclosure - through fear of them getting lost and scared. Or taken by someone else.
I do think the parents have been irresponsible.
I do also think that maybe the barriers should have been slightly better. But like others have said the barriers are there to keep the animals in, not humans out.
The parents should have made clear that the boy shouldn't have been scaling ANY fences. Regardless of whether it was a gorilla enclosure or a harmless animal (yes you could argue that no animal is harmless but that's another thread).
I don't think it's neglect but I don't think they have acted responsibly.
I do feel sorry for the gorilla. It only did what comes naturally to it.
I don't think any of us can say what it was going to do or whether it intended to hurt the boy because at the end of the day it's an animal and we can never be sure.
I'm sorry it died, but I would have been sorry if the boy died too.
I'd like to think there was a way around it but I'm not sure that there was.
Hopefully this incident will prevent things like this happening in future.