Thanks, ladies, for all your help. My friend (mother of one of the 10yos) thinks I should do this as a business. I said I'll see how it goes first with this batch of kids.
I have a rough outline. I've got, I think, 6 sessions of about 2.5-3 hours each.
Measure, discuss choosing pattern size (at least 2 girls will need to sew a much-lengthened "smaller" size), buy fabric, discuss pretreating fabric, demonstrate some seams and have them make samples.
Their "homework" will be to wash, dry, iron fabric. Mine will be to trace and prepare patterns. I'm considering cutting out two of the garments ahead of time, and then on the next session just have the one for me to explain layout and demonstrate how to cut the third garment. I think this will get them ready for the fun stuff.
Cut garment as mentioned above. They can stitch and I'll do any unpicking necessary at this stage. Teach them about hems and elastic casings.
Subsequent sessions. Continue as discussed, except that they can cut their own garment next time.
I think we should get at least two simple garments finished doing it this way. I will also find some nice trims, buttons, and one kid wants to make the flower on one of the skirts (the Simp).
I think that sounds doable. Any further thoughts?