The radish would probably be the best for your purposes . They grow fast and go to seeds in a few months , come in many colors and shapes and sizes. But stay away from hybrids unless you want unpredicted results .

Thank you farmfreedom!
Radishes certainly grow fast and I can easily imagine being able to do not just the two required generations but more. Great suggestion.

but...

What properties of radish (color, shape, etc) are there that are controlled by single genes that would be obvious to my 8 year old?
I have a general idea of how to cross pollinate flower (take a paint brush, brush the pollen from one flower and brush it on the next) -- is it just as simple for radishes? (I guess I have harvested any radish I have ever grown before allowing them to bolt/ go to flower)
While I have planted radish seeds many times, where to I find the seeds on a mature radish plant?

Arabidopsis thaliana is available from numerous sources. It's the geneticist's plant because it grows so quickly. There are numerous well-documented forms, including ones that differ in size.

My hybrid zinnias have bloomed as quickly as five weeks from seed. Some zinnias can start blooming when they are only 3 inches high. For me, zinnias are more fun than Arabidopsis or radishes.

ZM

radishes may mature in 19 days but they take about 54 days to get harvestable dry seeds . They have a "rats tail radish " that is grown for its seeds. Radishes come in red , white ,red&white, Black Spanish , and I have even grown purple by accident . They come in different sizes and shapes : globe, icicle , French breakfast , minowase, dakon .the roots get woody , then the tops grow into branches then they flower then they set pods like small pea pods which dry out and turn brown . then you harvest the seeds. As to single gene inheritance ? I am not sure .

If you need help on how to cross breed plants , check out one of Carol Deppes books:" Breeding Your own Vegetables " ,"Breeding Your own Vegetables for farmers" .