Oh I understand there are contingencies. That wasn't my point. My point was not all safety measure work all the time. All it takes is one incident to change everything. Look at this zoo incident. The enclosure had been there, in this same state, for how long? With no incidences. Until now. In this case all it took was the perfect combination of negligence on the part of the zoo and a parent who wasn't properly supervising there kid (and I understand what you mean about kids bolting off Izzy as I'll explain below, however from most accounts here the mother wasn't even paying attention to the child when the incident occurred, so there is a difference) for a fatal flaw in the enclosures design to become glaringly apparent. As sad as it is, these things do happen... at zoos, theme parks, water parks, ect. No incident that happens is ever not avoidable, and often they occur from issues that previously where not considered issues.
As far as young children are concerned, I know how difficult they can be to keep track of in places like this. They get excited, and when dealing with multiple, often have different ideas of where to go and what to do next. I'm not a father, but I am the 'favorite' uncle to my 4 nieces ages 5 to 10. I buy them all season passes to our local waterpark every year and take them through out the summer while my sister works. Call me overly cautious, but I have a rule that none of them go father then double arms reach from me at all times. They break that rule, and we all go home.