on TV the zoo said the gorilla was not shot with a tranquilizer because he was close to the child and a tranquilizer can take a few seconds to take effect. Also, when an animal is agitated it can take a bit longer to take effect, and there is no way of knowing just how much longer. Also, the dart hurts. So, you take an agitated gorilla who is holding on to a little boy and shoot him with something that causes him pain and agitates him further, and leads to some disorientation rather quickly, that rush of adrenaline in the animal could've made it even more dangerous for the child. They did wait for the gorilla to distance himself from the child to see if they could get a tranquilizer in, but the gorilla stayed right with the child the entire time. It was too risky they said.
As for the rest - even attentive parents can lose a determined child. My parents lost my brother a few times before they decided to not take him anywhere until he got older. They were pretty attentive parents, but I (The older sister) was a very compliant child who never ran away, and was not very determined to do anything, lol, so my brother's personality was polar opposite and unexpected. He'd promise to stay put and then sneak off. A couple of malls had to close the doors because my brother would take off. I remember vividly my mom running around the mall in tears looking for my brother. My mom was the type of parent if you weren't RIGHT by her side, she'd start calling for you. She'd hang on to your hand usually. But sometimes as a parent you have to let go to pay for stuff, or grab a tissue out of your purse or whatever. Some kids will take full advantage before you know what the heck just happened.
My oldest was like my brother, but she was my oldest so she had my undivided attention and she never took off (But she just about pulled off her arm trying to get away). My other 3 kids were like little ducklings and I never, ever had to worry about them wandering. A place like the zoo that is built for kids needs to be certain they are built for the most determined, non-compliant child. Even the most attentive parents have their slip ups, and pair that with a child driven towards something and a subpar enclosure, and you have a recipe for disaster. It's really a no brainer. I fault the zoo here. They put their animals at risk for assuming a crappy few wires would keep out all children always.