Potatoes will grow quickly but won't be ready to eat quickly. You can cut the "eyes" out of the fruit, eat the rest (kids love mashing!!), then plant the "eyes" with the nub side up. If all you're looking for is some top growth, this is quick, easy and was a hit with Cub Scouts.

Another idea is sunflower seeds, but there is always the problem with missing front teeth in 1st graders to crunch them open. Spoken from experience....