MY DS saw Lord of the Rings when he was 4 and became a big fan of all of the LOTR movies from then on. I would compare Avatar to that type of movie -- violent but make believe. I am somewhat picky about what I let the kids see and don't allow PG 13 for the nudity & dirty language kind of stuff, but the pretend stuff like this, the Harry Potter movies, Transformers, Spiderman (DS saw that one at 3 years old), I think are OK.
The question is, will your 6 yo sit through a 3 hour movie?