This may sound like a little bit of a cop-out, but a balanced diet is the key. Depending on how much they train, that balance tips more and more toward carbohydrates. If the athlete is doing 2-a-day workouts, they almost can't eat enough carbs. If you think about the time it takes to run a marathon, the kids can easily be swimming 2 marathons per day!
If I were picturing a plate, I would think a quarter of the plate would be protein, a quarter vegetables and half carbs. If you are a parent asking this, your plate should almost be the reverse of this, a quarter carbs, a quarter veggies and half protein. You could also eat on a salad plate while your swimmer ate on a serving platter!
All the typical caveats apply. Don't drink sodas with the meal. The very best thing they can drink is water with some lemon or lime squeezed in to it. Chocolate milk is better than sports drinks (and cheaper).