"Way back when," from 1911 to 1938, Lucy Fitch Perkins wrote a series of books that were beloved by youngsters--the "Twins" books. The protagonists, as one might guess from that description, were twin children, always a boy and a girl except in one volume. I remember Dana's Bookstore in Providence having some copies of the book, alas lost in their fire.
Recently, eight of the books have been transferred to e-book form and posted online, so I had the opportunity to read them. I also found the Twins' Homepage, which gives synopses of most of the books. I was quite interested by the description of Perkins' flouting the conventions of the time by making the girl character in almost all of the books just as ambitious and adventurous as her brother--while still keeping her feminine "values" as was required at the time.
The books about the younger twins are mostly nice little travelogues that contain details of child life in that society. The Dutch Twins (age 4), The Eskimo Twins (age 5), and The Japanese Twins (age 5) fall into this category. The adventures are very simple but fascinating, talking about traditions that had not yet been diluted by the influence of movies, radio, television and the internet.
As the children get older, the adventures get a bit more complicated as well. The Swiss Twins, age 9, and The Spartan Twins, age 10, both embark on journies which hold a little danger; the former are caught in an earthquake and have to get their sheep home by themselves, the latter overhear a plot to discredit a well-known man and are kidnapped when this is found out and must get away to warn the potential victim.
Two of the stories, The French Twins and The Belgian Twins, take place during World War I. The description of life in those societies, therefore, is interrupted by artillery and separation, even death. The scene in the former book, where the cathedral of Rheims is bombed by the Germans and the soldiers inside are killed, is very strong for a novel written for children.
My favorite of the eight books, however, is The Scotch Twins, a corking adventure story about Jean and Jock, who live with their father in a "wee hoosie" on the land of the laird. The twins and their friend Alan discover that a poacher is on the laird's property and set a trap for him, have adventures in a boat and in a secret cave--all like a jolly Enid Blyton or Swallows and Amazons type tale. Jean not only participates in all the boys' adventures but can keep house for her widowed father "as good as Mother." Plus there's quite a surprise at the end.
You can find the books at Project Gutenberg and also at Blackmask.com.