Does she want to be Fancy Mary Poppins (white dress?) or Mary with the black hat and carpet bag?
I made a Mary Poppins costume ie:black hat with flower, red coat, that she flys into town in.
Took a cheap red Halloween devil cape, cut the ends of sleeves and bottom, hemmed them, added buttons down the front on one side to make the coat. The rest of the stuff I got from goodwill or had- a ruffled white shirt underneath, long black skirt, black ankle boots, glued some fake flowers on a black felt hat, white gloves (which were actually white stretchy mittens).
I actually did make a carpet bag to carry along with an umbrella. For the bag I just bought maybe 1/2 yrd. of brocade fabric at
Walmart , folded in half, and sewn up the sides. Added a purse handle- done.
It was fabulous, everyone loved it, and it probably cost me about $25.
Just an idea if you want to go that way.