I do not believe in letting a child die, horribly, while I do nothing, if there is something I could do that has any reasonable chance of success.
You're entitled to do whatever you want in that hypothetical situation as long as you don't endanger anyone else. (Let's be honest: none of us will ever find ourselves in similar circumstances; this was a freak accident, with a set of facts not likely to ever be repeated.)
A very limited number of people (maybe 4 or 6?) were at the railing on the observation deck in a position to jump in immediately. At least one, and perhaps two, of them were probably the parents.
By jumping in, you'd be betting your life that gunshots will scare the dogs off. If that worked, you could more easily and safely do that from the observation deck. If you jump in and gunshots don't work, you are left next to a now-certainly-dead 2-year-old fighting for your life against a pack of up to 11 painted dogs. Best of luck to you, but I will not be putting myself in a similar situation by choice.
Insinuating that anyone who doesn't jump in "believes in letting a child die" is simply absurd.
Perhaps the zookeepers ought to have had guns, and ought to have been the ones to use them, since they were most familiar with the dogs' behavior, and most likely to take appropriate action if gunfire could have helped the situation.
Consider another plausible scenario: you decide to attempt to shoot at the dogs (but shoot wide of the child) from the observation deck. The two parents and others are probably screaming incoherently, "Help!", "Help our child!", etc. You're shooting at something. Suppose there's an off-duty anti-gun Philadelphia police officer close behind you, unaware specifically that a child has fallen into the exhibit, only aware that there's a child in danger, lots of screaming, and that you're shooting at something. I wonder what happens next.
Worst case, you could end up dead along with the 2-year-old, the parents feel even more guilty because you died trying to rescue their child, and maybe 3rd parties feel guilty or traumatized if they were involved or involved through inaction in your fate. While they'd be happy that you tried to rescue a child, your loved ones would have lost you. If you have kids yourself, would you jump into that exhibit at the risk of leaving your own children without a parent?