We used the dining plan on our last trip. DH, DD7 and myself were on the plan, and DS2.5 tagged along as a freebie. IMO, I wouldn't pay for the plan for a 2.5 year old. Our son can chow down with the best of them, and he never went hungry. The CS and TS portion sizes are huge, and there was more than enough food to go around. We usually ended up leaving food on all of our plates. All TS restaurants brought out an extra plate for DS without our having to request it. The most we ever did was purchase an extra drink for him at the CS meals.
In answer to your questions about being able to save dining plan credits, you won't be able to do that. All credits will expire on the last day of your trip.