The fire department says the blaze at 2912 Bruner Avenue in Wakefield broke out just before 4 p.m. Thursday.
Two children, ages 2 and 3, are fighting for their lives after being rescued by Good Samaritans.
Two 18-year-old's and a 60-year-old are hospitalized in serious but stable condition.
Eight firefighters also suffered minor injuries. It took a little less than an hour to bring the fire under control. There was no immediate word on its cause but the fire chief at the scene said, "There is a possibility that it was deliberate".