The woman, whose name was not immediately released, was 32 years old, Deputy St. Clair Coroner Curt Schildknecht said. He identified the children as a 9-year-old boy and two girls, ages 10 and 11.
Fire marshals were investigating the cause the fire, which had been reported shortly after midnight.
The blaze destroyed much of the roof and melted the awnings in the front of the house, leaving mostly a charred shell.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969