Earlier I mentioned about growing a plant from a potato, but I like your idea, Pat. Sweet potatoes also make a lovely plant that is easy to start in a cup with water. It is also a little more exotic that a plain ol' potato.
Anything where they can start the plant themselves, whether from a cutting, seeds, or even a potato, would be far more interesting to them, I bet. Anyone can just buy a plant and where is the fun, or accomplishment in that?