Start with a fabric grocery bag ($0.99 at the supermarket). Pick one that is a good size for your child, we used the smaller versions so they can still manage to carry it on Halloween night.
Decorate with acrylic paint in Halloween colours and encourage your child to make any design they want. We did this with 7 kindergarten girls & 1 preschool boy and the designs were varied. Some precise and others messy because as my daughter told me “Halloween is spooky so that means messy!!”.
Fabric paint would be even better for a more permanent design but I wasn’t that brave!
A piece of newspaper in the middle of the bag kept the paint from seeping through to the other side. Once dry the bags are ready to go! Such a productive form of artwork!