The parents probably had the kid sitting on the low fence watching and he might have squirmed out of their grasp after telling him how CUTE the gorilla was......either that or he crawled thru the rung of the fence....either way the parents are responsible for his safety.
Zoo enclosures to dangerous animals are usually set up with multiple barriers so the kid must have been unattended for some time to make it that far. Whoever was with him is responsible for the death of an endangered species.