It sounds like you are off to a great start. You have quite an age range. They could take off at varying degrees of interest. A book bag, simple pencil bag or a small wall hanging/doll quilt are also some straight line sewing ideas. Here is a photo of my 6 year old granddaughter sewing tiny pillow which she then learned to hand stitch the opening closed. I have been working with her for 3 years now. She handles the machine with Grandma sitting next to.
Years ago I taught a homeschool group of 6 girls ranging from 1st to 3rd grade. They loved it and have all taken off with sewing except my daughter (who was in the group). I started them off with drafting their own pattern out of newspapers of a butcher style apron. Things I stressed were raising the needle to its highest position before pulling the fabric out to cut the threads and holding onto the thread with a finger while they stitched their first two stitches when beginning to sew again. I had to hold my hands behind my back to keep myself from not reaching around them to do things myself. Verbal coaching is better than doing it for them.
You are doing a great thing for them. I enjoyed both teaching others to sew and homeschooling. Have fun!!!!
PS: I agree with Selm, hold off on the cleaning the machine. That could be used for those who take off faster than perhaps the younger ones. Also, an advanced skill would be to learn how to thread the machine themselves (stressing having the needle up in the highest position so the tension discs are separated when threading it) and then how to thread the bobbin.