My son's class (he's 4) grew green beans and/or lima beans. And Max's plant is doing great! We just transplanted it from the little styrofoam cup into a planter yesterday.

(I wanted to add the image here, but you can see it on my image gallery)

I think also sunflowers could work well. And basil can sprout up fast.

Personally, I had good luck getting cilantro to germinate quickly, but less luck getting it to be anything other than spindly... ( kids probably won't care about that)

Corn is also kind of fun. Big (and identifiable) seeds. Our silver queen corn that we planted this year took about 5 days to germinate. I imagine the toddlers could plant on Wednesday and by Monday see the corn stalk coming up.

ooh I have extra corn seeds leftover too. My husband's corn crop was a dismal failure, but I suppose for a first attempt, it's to be expected.

I got a bunch of corn from the farmstand we go to every week, and his class is going to have it for snack tomorrow. G is a master at shucking corn now, he'll be able to teach all his buddies :)