Another theory is that the two are elementals who will show Elsa that she isn't the only one in the world with that kind of power. It would make sense if they're the ones behind all the mysterious going ons in Arendelle, since they're in the forest that seems to be a huge focal point in the film. That would also be a perfect way to segue into the girl becoming the rumored girlfriend of Queen Elsa. Though — again — the two seem fairly young and neither could pass as old enough to date any of the main three.
Another theory on their identity is that the two are Disney's version of Gerda and Kai from the original The Snow Queen fairy tale that Frozen is based on. Obviously, elements of the story would have to change since Elsa is the Snow Queen and we know she's not evil, but it would be awesome if Disney decided to bring these characters into the fold rather than giving bits of their story to different characters (like Kristoff is bits of Kai). The brief interaction in the trailer fits the story of how Gerda and Kai were close friends and it would be cool to see how their story changes to fit Frozen's narrative. They could also just be a pair of kids who are dabbling in some dangerous magic that serves as a catalyst for all the chaos.