Police said one of the boys working the project Sunday may have tried to either start a fire in the fireplace or was testing the fuel, the Seattle Times reported.
The Arson and Bomb Squad found "several completed homemade fireworks bottle rockets, small aerial display mortars and a skyrocket" in the living room, police said.
Investigators also found chemicals for mixing up pyrotechnic powders.
Seattle Fire Lt. Sue Stangl said the boys were in serious condition when they were taken to a hospital for treatment of cuts and burns.
Seattle police Detective Renee Witt said the explosion broke out windows in the living room and kitchen and flung debris into the back yard but did not cause a fire, CNN reported.