The incident happened Tuesday at the Solfatara Crater in Pozzuoli, a popular tourist attraction near Naples.
Naples police told CNN the family of four was visiting from Turin, and the 11-year-old boy wandered into an area that is off-limits to visitors.
The Solfatara, a dormant volcano, emits sulfurous fumes. The area around it is known for a type of quicksand, which makes the ground unstable.
It’s not known if the boy lost consciousness because of the fumes or if the quicksand pulled him in. But when his parents tried to rush to his rescue, they too were were sucked in, police said.
Another child, 7, survived.
“I’ve been here for 40 years and such an accident has never happened,” Armando Guerriero, owner of a bar located near the entrance to the volcano, told the ANSA news agency.