Catapults featuring a plastic spoon are an easy and fun project. For the simple design shown devincollier.com you will need
- 9 craft sticks
- 4-6 rubber bands
- 1 plastic spoon
- some ammunition (mini-marshmallows or craft pom poms)
But a quick Google search will show many other designs that also include clothespins, pencils, corks, etc so you could provide your girls with a variety of materials and let their imaginations fly.