You can download the free 2T/3T pattern here. I say @2T/slash/3T because Sadie's kind of in the middle of the two sizes....the finished dimensions of this pattern are 21" around the waist, 8" around for the sleeves (though you can definitely use more elastic to get a wider opening), and 18" from collarbone to hem.
Adorable little dress. FREE pattern.
Just watched Princess Bride again and you wouldn't be able to tell the film is from 1987 by the costumes! I love her red dress-- the sleeves are to die for.