With their own machine be sure to go over the details of threading EVERY time they come and let them do a test sewing to see if anything is out of line. When they are out of your sight kids like to tinker with the knobs etc. and then you spend 30 minutes getting the machine back in order. I have taught 20 yr olds and I have this problem.
My 7yr grandson made a quilt 20"x24" in one day. Just to let him see what it was all about. Started with a 3"x6" then went around and around with 3" strips until it was as big as he wanted. It was a good way to practice stitching straight and handling the fabric. No it didn't lay down flat but made a quilt for his stuffed bear. Good luck. Kids are so much fun to work with