Snow White has two dresses during the movie and as I've learned from Hannah her doll is in theater and likes to act out her favorite movies (which coincidentally is Hannah's favorite movie, Snow White)
I used Kirsten's Party dress pattern for both costumes (crazy) altering Snow Whites main dress just a smidgen.
Download and print off the pattern here:
American Girl patterns
Once you've cut it out (I would add just a little extra onto each of the sides of Kirsten's tops and waist line (approximately 1/8 inch total) I learned the hard way that this particular pattern printed small.
For Snow Whites peasant dress:
a small light blue square (literally)
Thread (white and brown)
Light Blue ribbon for hair