There is no one food that is essential for good nutrition. Meat is an excellent source of protein, but the protein in Milk is just as good. Yogurt is ounce for ounce the equivalent of milk as a source of protein and calcium. Eggs and Fish provide complete protein as well. Even cereals and grains can provide adequate protein if they are taken in the right mix.
Protein is the essential ingredient for cell growth and repair.
A high protein diet tends to be high in fat since very few protein foods are low in fat. This includes pastas and cereals which are the kinds of foods most kids prefer. If you look at the Food Pyramid you can see that the base is made of cereals and grains, indicating that these should be the foundation of a healthy diet. Just remember that the amounts of food shown on this Pyramid are for adults. It is possible to be well nourished without eating vegetables.