The fence is there for a reason, the run between the fence and the moat is there for a reason, the moat is there for a reason.
The zoo had a enough safety. It just isn't "fool-proof" as you can tell because fools get through. I feel like when there's a fence there, it's obvious not to jump it. The kid should have known that no matter his age. And the time it takes for a little boy that age to climb over the fence (Short as it was, even still) and then the run through, the mother should have seen by then.