I don't see how this is the parent's fault in the slightest.
The ZOO failed, not the parents.
If you have to constantly watch to make sure your 4-year-old doesn't get into an enclosure with a dangerous animal at a zoo, then no parent in their right mind should take their kids to a zoo. It should be assumed that it's too dangerous for toddlers.
If it's a dumb ass teenager who scales a 25 foot wall on a dare from his friends? Then, yea. The gorilla can pummel him to death if it wishes and I won't shed a tear unless the Gorilla is put down.
But a 4-year-old toddler just climbing a couple feet and crawling the rest of the way? No way. The zoo failed. Plain as that.
You can't focus on just keeping the animals in the enclosures. You have to keep the people out of the enclosures too and assume they're not bright enough, stable enough or old enough, to know better.