It is June 1940, during the Battle of France. After five-year-old Paulette's parents and pet dog die in a German air attack on a column of refugees fleeing Paris, the traumatized child meets 10-year-old Michel DollĂŠ whose peasant family takes her in. She quickly becomes attached to Michel. The two attempt to cope with the death and destruction that surrounds them by secretly building a small cemetery among the ruins of an abandoned watermill, where they bury her dog and start to bury other animals, marking their graves with crosses stolen from a local graveyard, including one belonging to Michel's brother. Michel's father first suspects that Michel's brother's cross was stolen from the graveyard by his neighbour. Eventually, the father finds out that Michel has stolen the cross.
Meanwhile, the French gendarmes come to the DollĂŠ household in order to take Paulette. Michel cannot bear the thought of her leaving and tells his father that he would tell him where the stolen crosses are, but in return he should not give Paulette to the gendarmes. His father doesn't keep his promise: Michel destroys the crosses and Paulette ends up going to a Red Cross camp, but at the end of the movie is seen running away into a crowd of people in the Red Cross camp, crying for Michel and then for her mother.

Question: Why do Paulette and Michel build a cemetery? 
Answer: In order to cope with the death and destruction of the war.
Alternative answer: To bury pets in