I agree, some parents don't want bean plants in their home but I can't
think of anyone who doesn't want fresh basil in winter. Mint and
lemon balms are good too, most kids do a double take when they smell
these for the first time. Christmas pepper (get edible ones) and
sensitive plant (Mimosa pudica) are impressive too. You can also make
a sprout kit for lentils and alfalfa.

Don't forget vegetative propagation: african violet leaf, potato,
garlic, hens & chicks, etc. If you're really daring, you can start a
vermicomposter.

Here's some sites:

http://www.kidsgardening.com/
http://www.bry-backmanor.org/gardenfun/
http://www.global-garden.com.au/gardenkids.htm
http://www.growingedge.com/kids/
http://www.urbanext.uiuc.edu/firstgarden/
http://yucky.kids.discovery.com/noflash/worm/