Alice in Wonderland is a classic costume for little girls, and they are easily available from costume stores. If you'd prefer to make your own, you will need a pretty, old fashioned dress (preferably pale blue) and a white pinafore or apron. Tie a blue ribbon in the hair, and your costume is done.
If you like, you could stitch a small red heart and diamond, and a black club and spade cut from felt to the apron. Alice could also carry a white rabbit with her - perhaps a soft toy or our felt rabbit craft?