Dakot Blue Richards play the part of Lyra. The movie will premiere in December.

Dakota Blue Richards will voice Lyra in the upcoming video game about the film His Dark Materials: The  Golden Compass where Dakota plays Lyra, the main character. About voicing the game Dakota said: "I do find it quite difficult, but I also find it fun, in the same way that I found doing the green screen stuff fun [on the film set]"
According to http://dakota.skyefairy.net/ (where you can read the rest of the article) 

http://dakota.skyefairy.net/ , fansite for Dakota Blue Richards who star in the upcoming movie "His Dark Materials: The Golden Compass", has some newly added images from the official site of Dakota as Lyra, and some bits form an interview with Eva Green where she talks about Dakota.