I don't know about the legal problems that might ensue from discharging the firearm to scare off the dogs, but if someone had shot and killed any of them, I'm almost certain he or she would be hearing from the zoo's lawyers pertaining to "damage of property."
Having said that, human life is more valuable than some dogs and I think that shooting them to save the child would've been the pertinent course of action (assuming, of course, that you could get a clear shot and not put the child any further into harm's way). The only problem with that is if the child had been killed in the fall, not the subsequent attack, then the zoo could easily claim that killing the animals had not been justified, even though there was no way for the shooter to know that the child was already dead.
I have a mild interest in guns. Actually, I think the clinical term is "obsession," but that makes me sound like some kind of gun-nut. Which is fair, since I am.
Wastin' away my future children's inheritance one box of ammo, range fee, and bottle of Hoppe's #9 at a time.