I don't have younguns but I know someone who started their kid out on a machine (no thread in the needle) and lines on a piece of paper. This was to just learn how to control speed and improve accuracy. sounds like a smart way to go to begin. not sure what the next step would be though....
maybe work up to making a simple teddy bear blanket (made from a single square for the top and back, sew around the sides with a space to flip it inside out).