Judge Wassall told Truro Crown Court that Purcell had been part of a joint exercise to extort money from the occupants and that it had been to do with drugs.
He said it must have been an 'absolutely terrifying experience' for the three victims, in particular the boy, who watched his mother kicked and beaten, and her partner slashed with a knife.
Police said they are still hunting the rest of the gang but were pleased with the sentence meted out to Purcell.