You can tell the non parents on this thread, that said its one thing to lose sight of your kid, it's a completely different thing for a kid to have time to climb a three foot fence especially in a crowded public place where you should really have a hold of their hand
the Zoo keepers did the right thing they didn't have the benefit of time to think about it, it's a shame the gorilla had to die I lay that squarely at the hands of the parents.