My sister made an apron for a Halloween costume from blue and white checked fabric she got. She used a really simple pattern if you are into sewing... maybe each girl could get a Dorothy apron and you can braid their hair in pigtails at the party. In the kids section at the dollar store I got my daughter a cooking apron, but it was purple and white checked. You might be able to find something like that, that would work. Also, Easter time type Jelly shoes might work for ruby slippers if you can find them in red or pink. I am not sure where you can find them this time of year though. Even a bunch of red flip flops from Old navy for a dollar something a piece and let them sparkle them up as a craft if you want. The dollar stores definitely have cheap white tennis shoes for sure. They look like little Ked's brand shoes. If you got those, they would have more working space to add red sparkles or red paint and they could wear them over the winter. Target has "Rub Red Slippers" but they would not be cheap. You can get baskets inexpensive at yard sales, Christmas Tree Shoppes, and dollar stores. They do have a lot of stuffed animals at the dollar store right now too for Toto's :o) I loved that idea as well!
i normally try and make a lot of decorations. If you are into that, you could make stuff from construction paper and hang it on the walls. An old house with a swirly tornado taped beside it. Maybe some legs sticking out underneath the house to represent the wicked witch of the east? I saw large posters for cheap recently, I can't remember if it was at Michale's craft store or 5 Below maybe? if not, someone on here already mentioned printing things off the computer! Halloween witch decoration. Fall scarecrow decoration. Make the tin man out of grey paper.
Does your daughter like the good witch? There is a lot of stuff you could do, just princess pink sparkly things to represent her. A magic wand? The girls could make a hat like glenda's out of construction paper as a craft?
I know they have blue and white streamers at the dollar store. I have seen games where they race to make a mummy out of streamer paper, separating into teams and seeing who is done first. Maybe you could do that, but with a Dorothy apron. See who makes the apron best or fastest? i don't know how well it would work, but I bet the girls would think it was fun either way!
I hope you get a lot of ideas. I am going to check back and keep reading!