What would be a good plant to show the process of evolution to a small kid? Obviously it's a long term experiment, and preferably a fruit or a veg that is edible (like tomato?), and what characteristic would be (size, colour) good to select for?
This is for an animal, not a plant. But, when I was in school, the Peppered moth was used as an example of rapid evolution from the 19th century. Its white coat made it very noticeable against soot-stained surfaces so it had a rapid selection pressure to become black. If you can organise a trip with your kid to a natural history museum, you might be able to find some specimens.