NBC News is not naming the boy and his loved ones because they fear for their safety.
Among the entries, the boy drew pictures of tanks, burning buildings and helicopter gunships.
“You realize that he understood everything that happened and saw everything,” Sosnovsky said. “It’s terrible. When I first read the papers…it was almost impossible. Very heartbreaking.
Along with depictions of war, there are more innocent images. The boy has drawn an imaginary celebration for his upcoming birthday, complete with a cake and friends in party hats.
“I woke up and smiled yesterday,” he wrote. “My grandmother went to fetch water and came back. By the way, my birthday is soon.
But the reality of childhood in a besieged city erupted.
“My two dogs were killed, as well as my grandmother Halya,” he wrote. “And my favorite city, Mariupol, is dead.”