I agree w/ Ann, something they can finish rather quickly to keep up the interest... which advocates for getting the Mom to agree to using the machine. I think with the appropriate safety instruction a child can use a machine as safely as we can, maybe safer, b/c when they run into a problem they're going to ask you for help rather than try to solve it themselves, which is where we sometimes get into trouble (ouch!) Looks like you've got a nice machine that will have speed control. When I taught my son LY (then 9) DH put a block at the front of the foot pedal to act as a speed regulator and that was important, kids will naturally want to go pedal-to-the-metal
Maybe embellishing w/ some hand stitching would be cool too, to give them some experience using a needle & thread....
Florence