I brought my 4 1/2 and almst 2 year old boys there a few weeks ago on one of the days of our trip when my husband was at a conference. Initally, they were very interested in all there was to see, especially the aquariums. After we were seated in the gorilla section, things seemed to be going well, but after the gorillas started their thing, my older son became very afraid to the point where he was crying. He was actually shaking. He has never been like that before. After the gorillas settled down, as did he. Since we hadn't placed our food order yet, the waitress gave us the choice to sit away from the gorillas, which we did. I gave my son the option to leave or stay, and he said that he would be okay at the new table, and that he really wanted to stay as the new table was right by an aquarium. Well, after we had ordered our food and it came out to us, the thunderstorm part went off, with all of the monkeys screeching, etc. and my son became frightened again. We ended up having the rest of our food packed to go. The staff was wonderful there, and after I apologized for all of the craziness, the waitress told me that kids getting scared actually is quite common there. In all of this, my younger son was completely oblivious. Not to say that your youngest would react the same as my older son, but its something to be aware of. I hadnt even considered the fear potential, thinking that the kids would think it was really cool (which it was), but my son definately caught me by surprise, as he tends to be quite fearless.
On a side note, the menu had lots of really nice options, and, I enjoyed my huge Cobb Salad, both when I was eating it in the restaurant, and after it turned into unexpected take out.