Agree that pasta can be counter productive for some. Wheat intolerance is often the reason for this. The younger sibling you mentioned....quite typical..individuals often crave the food that they are intolerant to.
Would like to mention that for any parents who aren't restricting pasta, avoid for evening meals, particulary the night before the tests. The burst of energy is more likely to keep your child awake and won't last until the next day when it is really needed.
Hi ButtyBoo, well said - you definately need a snack if the tests run over a couple of hours. I think we gave our daughter a piece of fruit and a sticky bun (oh dear, now I've made her sound like an elephant