That is a wide age range! Think about a variety of activities for the different ages. Something that a 3 year old would enjoy will bore a teenager to death! Coloring pages of quilt designs is a good activity for the little ones. Sewing a block is a good activity for older pre-teens, maybe 8 and up. Design activities could work for older children. Appliqué by hand might be a good option for teens though you might want to make sure there is a machine option too. I have done "survival sewing" with teens - hems, buttons, and seam repair. It is particularly nice if they take home a little sewing kit - needle, a bit of thread.
Pam