The gorilla doesn't really look like he wants to hurt the kid. Lets face it, if a silverback wanted to kill a 4 year old, he could do it in about 5 seconds flat.
Thats the problem - it could have all gone badly wrong so fast. Made me sad though as it looked at face value as if the gorilla's instincts were protective.
Without more information I'm kind of inclined to blame the parents - I'd be absolutely on the ball if I was around potentially dangerous animals with young children of my own - sure sometimes momentary lapses happen but I'd bet it wasn't really the case here.