In the book Lyra has a more complicated plan on how to get the children out, it's alluded to when she tells Roger to have the kids have their warm clothes ready in the movie. In the book she's at Bolvangar for a couple of days before she's caught and taken for intercision. However in the movie they have to rush things, clearly, so plans get sped up.

Book storyline she plans a fire alarm. There is a fire drill while she's there and she sees that the adults aren't really organized for one and she can use that against them. So she starts a fire in the kitchen hits the alarm and they all run.

We had the Inkheart trailer as well, I have that book and now really have to read it before the movie :)