A S'more sounds so creative, you HAVE to make it!
When my daughter was growing up we worked with what we had and bought accessories if needed. When she was 8 I bought her a beautiful iridescent white dress for the holidays, and the following Halloween she was a Fairy, the dress had gotten short, we put tights under it, her ballet slippers, a wire tinsel halo in her hair and she carried a wand, our only expense, a dowel at the hardware store that we sanded down, painted and sprinkled with glitter. One year someone gave me a clown costume, she wore it with big shoes, we purchased an inexpensive makeup kit, and found a multicolored wig for next to nothing, voila, and she won for best costume in her grade. We did stuff like this every year, I never spent a lot on things she'd only wear the one day..
With the little guy I bought a giraffe costume at Children's Orchard for $6 two years ago, last year I found a Chewbacca one there for about the same price. He'll be 2.5 this year, and I plan to make his costume, not sure what yet, possibly he'll be a Hershey bar ; )
I say, take the time to look online for children's costume ideas, ask your kids which ones they like, and make it into a family time making them together over a weekend, and cherish the memories!