WARSAW, Poland—A second child of an Afghan family evacuated from Kabul to Poland died Friday after eating soup containing death cap mushrooms, which the family had unknowingly gathered in a Polish forest outside their quarantine center. The 6-year-old boy had received an emergency liver transplant but doctors were unable to save him. His 5-year-old brother was pronounced dead on Thursday at Poland’s main children’s hospital, where both were treated. The boys’ 17-year-old sister was treated at the hospital and released, in good condition. Doctors said the dose of toxins was less damaging to an adult with larger body mass than to children. Authorities are investigating whether negligence could have been a factor in the poisoning last week. The family of two adults and four children allegedly cooked soup with the highly poisonous mushrooms they found in the forest around a center where they were undergoing a mandatory quarantine. They entered …