You might want to meet the girls and then decide if you should teach the younger ones separate from the older ones.
It may well be that the older ones would be kind and help the younger ones in their learning ... and by doing that the older girls will really learn how to sew.
If you have samples of 3 different things they could make ... like a nap quilt, a pillowcase, or a tote bag then you could let the choose which they would want to do. If they do something the like then they will have better memories and probably be 'hooked' on sewing more easily.
ali