Sadly, one seed will only produce one plant. I don't know of any food-bearing plant that will grow to maturity in a plastic cup, except perhaps a single radish. However, you can grow 3 to 5 bean plants in a 2-gallon container.
I usually recommend dried beans from the grocery store for very young beginning gardeners. They're inexpensive, sprout quickly, and the beans are large enough for small fingers to handle. Just buy a bag like you would use to cook for your family. Many other seeds from fruits and veggies you buy to eat can be used to grow plants, but beans are the easiest and fastest, in my opinion.

"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams