I was 8 when I learned to sew. My best friend and I made Barbie clothes. Still have a couple - they were beautiful at the time, now they look hysterical. When we did the Girl Scouts they were 10 and we made a pillow. The co-leader and I felt they were getting too old for dolls and they all had a bed to put the pillow on. They weren't very big. The next year we made tote bags.
If they want to learn to quilt, my suggestion would be a patchwork using 5 or 6 inch squares that you have already cut out. If you know their favorite colors then that will help. Figure out how big you want it to be and then have them sew the squares together. You can iron the strips as they finish. Do you use the glue method? I have started using the glue stick to match the seams and the Clover clips to let the glue dry and I then remove the clips as I go along - that way there are no pins. Then layer the quilt to birth it instead of binding it. Will their mom be there to help?