Kelly,
You can find more information about each of the Disney Dining Plan options
here. The exact cost of the dining plan varies depending on what plan you choose, what time or year you visit, and the ages of the guests. Child dining plans vary from about $12-$22 per night, and Adult plans from about $35-$80 per night. These are rough estimates of pricing, current as of July 2011.
When you purchase a dining plan as part of your Disney vacation package, your meals are calculated on the number of nights you will be staying, not the number of days you will be in the parks. You could, for example, have 10 ay park tickets but a 6 night hotel stay, and then you would get 6 days worth of meal credits.
The dining plan is only available for guests aged three years and older. Because your son is two, he is welcome to eat for free with you at any buffet or family-style restaurant on property. If you are at a table service or quick service location and would like to purchase him a child's meal, you are welcome to do so out-of-pocket.
Hope that helps!