You got it! The atmosphere and mood of the film with your arrangement that is. I like the slight variation in the melody on the flute, and I like the tone of it as well. Thank God for those wrong notes when it comes to writing. Often times a slip of the finger can bring great inspiration and direction, or at least it adds a bit of flavor. In this case, this piece has its own unique flavor where it doesn’t sound like an exact copy of Elfman’s work, but it still has some quality that is reminiscent of the film. Nice job in maintaining that balance.
Adding more instruments? I think this is more than adequate and right for being a lullaby as this gives it an intimate setting.
Does Edward sleep?