Proteins, in addition to being the "building blocks" that build and renew organ and tissue cells, are the main components of the internal organs, muscles and skin, contributing to the production of certain enzymes and hormones and having an immune function (antibodies are proteins) and providing energy.
Those proteins that contain all essential amino acids (such as those found in meat, fish, eggs, milk and cheese) are called high biological value proteins, or noble proteins. Those that contain only a few, such as vegetable proteins (found in legumes) are defined as low biological value proteins.
If not enough proteins are introduced, the body must use muscle proteins to meet the requirements. Therefore, they are essential in the diet, especially during growth and development. On the other hand, the body is not able to store protein, and so all excess protein is converted into fat, promoting MS processes: it is best to introduce the right quantities of food, avoiding going overboard with those of animal origin.

Carbohydrates are an ideal source of energy with respect to protein and fat, as the glucose content is rapidly used for energy purposes.
Carbohydrates are distinguished either as simple carbohydrates (like the sugars found in fruit and milk, and sweeteners such as sugar and honey), that are digested and assimilated quickly and provide energy for immediate consumption, and complex carbohydrates (such as starches found in flour, grains, potatoes, etc.), which are absorbed slowly and provide energy to be used over the longer term. The energy supplied by carbohydrates is used for the functioning of the nervous system and red blood cells, as well as physical activity. Again, it is necessary to balance the intake of carbohydrates with that of other nutrients, as carbs create unnecessary excess reserves in the form of fat.

Fats, divided into saturated (from animal sources, such as butter and cream) and unsaturated (of plant origin, such as olive oil and seeds), are essential because, in addition to providing energy that is stored, they regulate metabolic exchange, carry some vitamins, and contribute to the formation of cell membranes of the central nervous system, brain and retina.
While carrying out important tasks, especially during growth, fat must be introduced in the right quantities, preferring unsaturated fats (too much saturated fat can cause overweight or even obese children and the emergence in adulthood of heart disease).
You should keep in mind that there are invisible sources of fat: a lot of saturated fat, for example, is "hidden" in cheese, eggs, meat, sausages etc., while unsaturated fats are found in fish, nuts and some meats.
During the first year, 85-90% of the calories taken in are used to maintain bodily functions and for growth, while only 10-15% are for physical activity. In the second half, proteins should provide 10% of the contribution of energy, carbohydrates 50% and fat 40%.

Vitamins are invaluable as they regulate and coordinate cell activity. They are mostly introduced through food. They are divided into fat-soluble, meaning soluble in fat (vitamins A, D, E, K), contained mainly in foods of animal origin, and water soluble (group B vitamins, folic acid, vitamin C, PP and H), contained in fruits and vegetables. Fat-soluble vitamins can be stored in the body, while water-soluble are not accumulated and therefore must be taken regularly.

Minerals, which are calorie-free, are essential to a child's growing body: calcium, phosphorus and fluoride contribute to the construction of bones and teeth, iron for red blood cell formation, sodium, potassium and chlorine regulate salt and water balance and magnesium and copper start metabolic cycles.
These are eliminated daily through sweat, urine and faeces, and must be introduced through food.