In the past Zoo's kept animals in small caged environments. Animal rights
people demanded Zoos create open envioroments for the public to view
animals in more of an natural state. To their credit they did create
them, but it would seem in trying to make the experience good for
viewing they forgot to protect the ignorant from themselves. Humans
are curious, and when you add that to lack of experience there can be
trouble.
There is no perfect solution, but I think a 3ft high barrier is no barrier
at all, at least to a determined human child.
So the fault is likely more the Zoo or the designers of it. I would blame
the parent, but she only acted like most all parents, and society doesn't
feel that should be changed.