You have gotten lots of great suggestions. Here's a quick one I just did with my 2 grandkids. A pillow was pretty quick and then I taught my grandson the art of pulling the stuffing apart and stuffing the pillow. His desire was make to make a pillow for his beloved cat but I know he uses it too. My DGD wanted to make a pillow for her dolls.
I did start kids out with straight line aprons. In that I also taught them to make their own patterns out of newspaper. Simple shorts can be a pretty interesting project.