Yes if she was holding his hand maybe he wouldn't have ended up in the enclosure. But he could have gone over/through the fence in a split second. And the zoo should have fencing which makes this impossible, surely?
The parents aren't being prosecuted for negligence and yet the Internet seem angry at the mother.
But then the people who think that also seem to think a tranquilliser works instantly, and the gorilla wouldn't react to the pain or impact of the dart.
It's sad and terrible though. For everyone.