I'm pretty sure that I read upthread that one of the closest witnesses said that his mom had no chance of stopping him, nor did she. The mom was occupied with other kids, and the incident took very little time at all.
So, I don't think the mom is to blame here. It's as if any child surpassed a barrier when her/his mom was focused on dealing with other children. I don't blame the child, as kids are as curious or as adventurous as they can be. So that leaves me with the zoo, which is not only responsible for thousands of animal species. Why not protect both equally?
I mean, would you put your money or valuables into an enclosure that a 4 year old could breach? And if breaching that threshold meant destroying the valuables, would you think that was ok?
In this case, a 4 year old breached a threshold that resulted in (what I consider) something beyond monetary value being destroyed. I think that says something about those who designed the structure meant to protect the gorillas. And that's why I do not blame the child's parents, or the people who shot the gorilla.