I remembered making hair scrunchies in a class in high school. It's a good easy project for a kid and you can make many of them! Don't forget to backstitch so your stitching doesn't come apart.
There are plenty of online places to buy fabric or I went to JoAnne Fabrics and just picked out a bunch of different Fat Quarters and Fabric Quarters. They are 18" x 21" and great for small projects.
For a child size scrunchy cut it 3" x 12". Cut the elastic 5 1/2" long.
For an adult cut it 4" x 16". Cut the elastic 7" long.