Kids have high demand for growth. Every body tissue needs calories for its growth. . That comes from sugars; fat for its metabolism and energy stores.. that comes fron oils.. aminoacids for tissue and dna building.. that comes from proteins.
A lots of micronutrients are required in small amounts for various body functions, enzymes, chemical reactions, metabolism, dna synthesis and neurological growth too.
Each body cell is like a small factory. . Whose heart is dna in the nucleus which is made up of aminoacids linked with sugar and phosphate bonds. .
Each cell has a kitchen called mitochondria which has energy store source where most metabolism occurs and it along with golgi bodies intitiates productions of various special proteins and enzymes for body functions. All including cell wall needs regular nutrienta like water calories carbohydrates proteins and fat.
To make a cell survive, proper nutrition is needed. Faulty diet can make every cell deficient for the nutrient.
Thus balanced diet is need for cellular level nutrition which will be key to health.