To do so would risk deficiencies. Your greatest nutrient need is the need for energy. Whole grain complex carbohydrates also provide lots of B vitamins, vitamin E, minerals like zinc and magnesium, plus fiber. Both provide fairly similar nutrition, so if your child isn't keen about veggies, serve fruits instead. Vegetables provide vitamin A, vitamin C, folate, some calcium and some iron, plus potassium.
Fruits provide lots of vitamin A and vitamin C, some B vitamins, and potassium.
Both are naturally low in calories and high in fiber. Pick the nonfat or lowfat varieties. Choose 2 to 3 servings a day. They supply calories, but little in the way of vitamins or minerals. Use them sparingly and watch to see where they have been added to food in the other groups, so you can learn to avoid them.