From MTV:

MTV News: Obviously the kids are key to this one, and we think about Spielberg working with kids. Were you naturally always comfortable working with kids. Did you rely on Steven in terms of working with younger actors?

Abrams: I don't think working with actors, young or old, is ever very easy or a given. Everyone's different. There are some actors with whom I feel I have this crazy psychic connection and I don't feel like I have to say anything and they get what it is. When you're casting, the most important thing is casting people who don't necessarily have to be told what to do at every turn but will inspire you by coming up with things you never would have thought of. Working with kids is something I had done only sparingly. In "Star Trek," there was a young Kirk and a young Spock. But never really like this, and frankly I was terrified. I was dealing with two main actors who had never been cast before and never been on a set before. They didn't know the most fundamental things about how a crew works. Everything was an alien experience for them. The fun of working with them, partly, is they weren't professional kids. And then there was Elle Fanning, who's from another planet herself. There's a kind of innate brilliance and sophistication, the likes of which I've never seen in an actor, especially an actor who when I worked with her was 12.

Check out the entire interview.