Elizabeth Swann is the first character we meet in Pirates of the Caribbean: The Curse of the Black Pearl. She is a young girl who is fascinated by pirates and the sea and also the daughter of the Governor. She is well dressed in what appears could be a dress made of silk with a floral pattern on it.
Here in the front you can see the front seamlines curving down into a "V" at the waist. Her sleeves are trimmed with wide lace which was very fashionable in this period.
Here you can see the details of the medallion:
McCall's 6139 which would need some adjusting on the seam placements and the neckline shape. This would not be a historically accurate pattern but it does come in children and adult sizes so that's nice if you are simply looking for a quick easy costume! There is also a pattern from Mill Farm for a girl's 18th century gown that can be found here. Sense and Sensibility Patterns has a Portrait Dress Pattern that comes in both adult and children's sizes that could be altered to work as well. You can also purchase a replica of the pirate medallion here.