Maybe fusible applique? Start them out by drawing squares and circles on paper, then sew on them without any thread in the needle. When they catch on, trace squares and circles and fuse them to blocks and let them stitch them using a blanket stitch. Teach them to go slow and steady, and on a larger circle, they shouldn't have to stop and adjust very often. If you use a varigated thread that matches the applique and background, it will hide the little boo boo's too :wink:
They could have a very cute cuddle quilt quickly with this pattern. A 10 or 12" block with a 8 or 10" circle or square in the center would work up fast.
You could help them sew the blocks together, and do a simple SID with a wider zig zag stitch. Or use a serpentine stitch, so it they wander a bit, it doesn't show up as much.