My boys are 2 1/2 and 5 1/2. We saw it yesterday. Both boys love the experience of sitting in a theater, having a cup of sprite (only special occasions), sharing some popcorn, and watching a big screen, so their attention span is good. They both loved the movie (I did too, actually--it's a beautiful place, footage you don't really get to see in real life, and I thought the cinematography was excellent). We went to the first show in the morning, before lunch, on a week day so we had the place to ourselves and I could quietly comment or lead them to think about it (comments, questions) as they were watching it. Glad we went! The boys love monkeys.
My 2 year old did well, but he did start getting a little fidgety the last 20 minutes because it was fairly long, but I just kept his popcorn cup full and broke out the "emergency" skittles. That keeps him seated, quiet, and happy during a movie and I strongly suggest having that little emergency stash if it's too long.
If you're cool with possibly not seeing the "whole" movie, another option I used to do with my first child when he was really little was taking a bathroom break and walk down to the arcade and play a game, then walk back to finish the rest of the movie.