first, how to thread the needle, set the bobbin and tension, and generally acquaint them with the sewing machine.
Second, choose a simple pattern that they can use to learn to sew straight seams.
I like a 4 patch (checker board) for a first quilt. It acquaints them with cutting, sewing straight seams and matching intersecting seams up. A jelly roll strip quilt is good candidate too, because it is all straight stitching. However, the Jelly roll doesn't hold a beginners' interest very well, I guess because it's a boring process, and you really don't see any results until you're about half way through making the top. The 4 patch gives a learner a lot of quick feedback and they can see the results of their work and make changes in their sewing habits as they go. It's also easier to seam rip a 4 patch.
Good luck and have fun.
Teach them to clean the machine frequently. My first adult sewing machine was so jammed with dust that it literally stopped sewing. I thought I was going to have to recycle it. Then it dawned on me that perhaps the machine needed cleaning. It sure did. The dust was packed into the feed dogs so tightly that it was like strips of felt!
My Mom taught me the little I knew about sewing when I started quilting, but I don't ever recall that she showed me how or why I needed to clean the machine! She probably did show me, but as a kid I probably tuned out.