The human/Na'Vi dynamics are changing throughout the entire course of the movie. And as for what has taken place before the movie starts... what do we really need to be told? We know the humans come from Earth, so it's clear that they are newcomers on the Na'Vi's turf. We know they've made peaceful attempts to connect with them, hence the schools and other "goodwill" efforts. We know the school was shut down and the general attitude of the humans and Na'Vi toward each other is precarious at best, so clearly something went wrong with those peaceful negotiations they were trying to have. Because the Na'Vi don't want anything the humans have to offer. None of this is a mystery. Like you said, the story is written for 3rd graders.

Now, the character development thing, I agree on. That tends to be my main beef with these archetypal stories. Just because everyone knows what purpose a character is supposed to serve, it's not a free pass for the screenwriters to gloss over character development. I could have used more depth from pretty much everybody in the movie.