From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156560-878630
Question: What can I plant with my 5-year-old that is easy to grow and
low-maintenance?
Answer: Choose things that are quick to germinate and grow or that
have sensory delights for children. Radishes germinate easily, mature
quickly (about 20 to 25 days), and kids seem to love to pull them out
of the ground. There's an 'Easter Egg' blend with round radishes in an
assortment of colors. Sweet peas are extremely fragrant and pretty.
(Soak the seeds in water overnight for easier germination.) Sunflowers
and hollyhocks are magical. You might want to use some transplants
also. Try bedding plants such as lamb's ears (which are soft and
wooly), pansies, and Johnny jump-ups (their little faces are fun).
Chocolate mint is an easy-to-grow herb. In fact, all mints are so easy
to grow and invasive that they are best planted in containers
submerged in the garden (unless you want them to take over). Marigolds
and cherry tomatoes are other good choices.