It seems quite real. The kids know their father is recording them, maybe that he wants their reaction to what they're about to see. You have to take that consciousness of being recorded into account.

The little boy gasps with a look of real surprise in his eyes and then looks at the camera, at which point he does exaggerate the gaping mouth for a second, perhaps for the advantage of his father. That is hardly mugging for the camera. That distraction from the action on screen quickly melts away as the little boy then has a genuine look of disgust, not mock surprise. You see his face turn serious, his eyebrows furrow slightly. You can see him hoping that Vader is lying and then his face accepts the truth.