The Bobbsey Twins or Merry Days Indoors and Out by Laura Lee Hope

According to Wikipedia: Laura Lee Hope is a pseudonym used by the Stratemeyer Syndicate for the Bobbsey Twins and several other series of childrens novels. Actual writers taking up the pen of Laura Lee Hope include Edward Stratemeyer, Howard and Lilian Garis, Elizabeth Ward, Harriet (Stratemeyer) Adams, and Nancy Axelrad. Laura Lee Hope was first used as a pseudonym in 1904 for the debut of the Bobbsey Twins. Series: The Bobbsey Twins (1904-), The Outdoor Girls (23 vols. 1913-1933), The Moving Picture Girls (7 vols. 1914-1916), Bunny Brown (20 vols. 1916-1931), Six Little Bunkers (14 vols. 1918-1930), Make Believe Stories (12 vols. c. 1920-1923), Blythe Girls (12 vols. 1925-1932).