@jupiter-sun-of-sweater-town group
@CrackpipeDreamer: Children's clothing throughout most of the Victorian era was pretty much just scaled-down versions of adult clothes. For a 12-year-old girl, she would essentially be dressed as a tiny adult, almost identical to her mother but with a shorter hemline. This would mean very full skirts, as was the style of the decade. Of course there would be plenty of petticoats, and if your character is later in the 1850s, she might even have a hoop or cage crinoline.
Her hair would begin to be grown long so it could be worn in adult hairstyles. She would also start wearing adult bonnets. (As you can see, this age was a milestone as the girls transitioned to adulthood.)