I don't believe there should be formal charges against either the mom or the zoo, but I don't think this is a situation with no fault on both sides. One of the early articles I read was that the mom had 6 children with her and was having a hard time keeping them all in line. I also read that the 3-4 year old had just thrown a challenge at her telling her he was going to climb down there. I understand kids can move before you can blink and these things can happen, BUT the mother needs to somehow be made to acknowledge that she didn't make the smart choice by taking too many children at once. No one person can handle 6 children like that. I don't know what the response should be, maybe even something as a simple note saying, "I was being irresponsible by agreeing to take too many children into my care at once." SOMETHING.
I also don't think the zoo should be charged, but again, they are not at no fault. Their barriers were inadequate. At the very least they should be fined the cost of installing fail proof separators from the public and gorillas and perhaps throughout the zoo.
I was sad and happy at the same time to see that in the middle of the crisis, the right call was made. Humans are ALWAYS more valuable than animals no matter how regal an animal may be.