Tough to argue either side of this. 50/50 liability split? Mother never should have let it happen, but safety measures should be in place for accidents. One of those tragedy things, mother definitely has contributory negligence here, how much though, who knows. Msg to any business though, not to mention one that can be hazardous, dummy proof the crap out of everything, otherwise some idiot will find a way to let something happen and sue you anyway. Can't blame the kid for doing what kids do, which is the primary reason the zoo should have some responsibility, because accidents happen regardless of parents negligence when it comes to kids, and therefore safety measures gotta be there in absense of common sense.
Side note, if my kid fell in there and wasn't hurt, I wouldn't sue, great story and good nickname for the little guy. If my kid was hurt, I'd sue if they didn't pay the medical bills and associated costs, but I bet the business would hopefully voluntarily pay those, unless my kid was really traumatized I wouldn't go for any pain and suffering. That's what insurance is there for, to settle for things that are necessary. It's the ones that sue when there are no actual damages that unfortunately ruin things for everyone else. I hope we hear more about what happens in that respect.