If you're looking for almost-instant gratification, radishes germinate very quickly, and you'll be able to harvest/eat them within about 6 weeks. When our son was 3 or 4, he loved being able to see things progress that quickly.
If you're using a clear container, plant the seed against one side of the container, and you can see the whole process - something else our son liked a lot.
As far as bugs, we did a lot of observation when our son was younger. He really liked watching ants and was able to deduce a lot from watching their behavior over the course of several months.
Tara
