My advice: Don't go it alone. My guild worked with a girl scout troop a few years ago about the same age. Many had never seen a sewing machine so we had one adult for each girl. Those adults need to know how to sew and know their way around the sewing machine (we all brought our own machines). We did a simple 9 patch with 2.5 inch squares for a mug rug. We precut the fabric and batting to save time. Started with a lesson on sewing machine safety and no one got hurt. It helps to have a machine with adjustable speed because they will be lead-footed.
Make it fun for the girls and praise everybody's efforts. Hopefully some of them will fall in love with quilting.