I've seen this twice now and there are several ways to view it - 2D, 3D or IMAX 3D. The rating of PG-13 is due to the violence (not language or nudity). After all, there are battles with Orcs, giant spiders, and elves. Legolas appears in this episode, patrolling the Mirkwood alongside Tauriel and the rest of the guard. When he searches Gloin, we see a portrait of Gimli - whom Legolas describes as a mutant goblin. The audience gets a laugh out of that, since we all know that Gimli and Legolas become friends during the Fellowship of the Ring. It also is amusing to note that Gimli calls Legolas "lad" in those movies, but here we see that Legolas is already grown while Gimli is still a child.
Parts of the movie that will probably stand out in your memory are the dwarfs floating downstream in barrels, the speed and grace of the woodland elves as they fight the giant spiders and Orcs, Bilbo tiptoeing through the halls of Erebor, the dragon waking in his piles of gold, and Gandalf trying to hold back the darkness when he confronts the evil at Dol Guldur.
Viewers who have already seen the earlier films will know what to expect from the incredible special effects and mood-enhancing soundtrack. Younger children might be frightened by the monsters or the fighting, or they may be accustomed to those sorts of fantasy elements.