The Bobbsey Twins or Merry Days Indoors and Out

The Bobbsey Twins are the principal characters of what was, for many years, the Stratemeyer Syndicate's longest-running series of children's novels, penned under the pseudonym Laura Lee Hope. The first of 72 books was published in 1904, the last in 1979. The books related the adventures of the children of the middle-class Bobbsey family, which included two sets of fraternal twins: Bert and Nan, who were 12 years old, and Flossie and Freddie, who were six. (Summary by Wikipedia)