Our small and large intestines secrete the enzyme lactase to digest the lactose in dairy products. Its secretion should be consistent. Only then will the lactase be broken down and the nutrients will be absorbed into the body and the wastes will be separated and passed out through the rectum. Some children have low lactase enzyme secretion. Some children do not secrete it at all. There are various reasons for this.
Some people have hereditary lactase deficiency. Children born to them also have this defect. Due to this, they avoid dairy products during their lifetime. Lactase secretion is also impaired in some preterm infants. They are allergic to dairy products. This condition can also be caused by bad bacteria in the intestines of children. Even if there is an accident in the small intestine, this allergy is likely to occur.
Many African American black children have allergies to dairy products. Within a few minutes of eating dairy products, they develop gas in the stomach and the stomach swells. Vomiting occurs. Similarly Asian children are also found to have this defect due to their genetics. Some communities avoid consuming milk from animals such as goats, cows, buffaloes, and camels. Due to this, they are genetically allergic to animal milk and milk products. 
A human can live healthy without the lactose in milk. Also increasing lactase enzyme secretion in the intestine through treatment is impossible with current medicine. So it is better for people with dairy allergy to reduce their intake of dairy products throughout their life.
Although these kids crave cheese pizza, cheese burgers, cheese sandwiches, and more, it is healthy to eat very little cheese. Brittle can be eaten with jam instead of butter. You can have other sweets instead of milk sweets. In tropical countries like India, eating buttermilk in the scorching sun during summer is cooling to the body. They can eat young water and nungu instead of buttermilk.