The Boxcar Children

The Boxcar Children was initially written in 1924, but the version that everyone now knows was published in 1942, and was specifically aimed at young readers.

The book chronicles the adventures of a family of runaway orphans, Henry, Jessie, Violet, and Benny Alden, fleeing their grandfather, who they believe to be a cruel man. They eventually find shelter in an abandoned boxcar and make it their home for several months, having various adventures, before eventually being found out and returned to their grandfather's custody. He turns out to be very nice, as well as filthy rich.

After many requests, Warner followed up the book with eighteen sequels, mostly mysteries, over which the characters gradually aged. Benny, 5 in the initial book, was 11 and working as a stock boy in a department store in book #19.

Then in 1991, the Albert Whitman & Co. publishing company decided to cash in on their continuing popularity. There are now over 100 books in the series, and counting. And a cookbook, that somehow takes recipes mentioned in passing in the books and... turns them over to children.

Not to be confused with The Railway Children.

Tropes used in The Boxcar Children include:

And Knowing Is Half the Battle: Every mystery book contained some minor science lesson, such as about plankton, or Captain Cook's voyages, or similar. A noteworthy one: Henry's sage observation that "Mashed potatoes don't smell." (The Lighthouse Mystery.)
Big Eater: Benny, whose catchphrase was practically "I'm hungry!"
Comic Book Time: Initially set in the depression era, more recent books have included passenger jets, tropical cruises, and the internet. They have a different summer adventure in every book, and they're all still young somehow...
Crane Game Gag: A variant exists with gumballs in The Pizza Mystery. Benny uses pennies to get a silver gumball at a gas station. A silver gumball can be traded in for a free treat. He ends up not getting any on the first four, but gets the fifth when a random stranger puts in a penny and shoves his inside. Because said stranger is a jerk, however, Benny refuses to get a treat, looking upset. At the end of the book, he wins a gumball on his first try, but insists on keeping it as his treat and souvenir for the trip.
Defeat Means Friendship: If the culprit of whatever mystery they're solving has any decency in him/her at all, he'll be reconciled to the Aldens right away.
Four-Temperament Ensemble: Benny is Sanguine, Violet is Melancholic, Jessie is Phlegmatic, and Henry is Choleric.
Orphan's Ordeal: The whole plot of the first book.
Parental Abandonment
Raised by Grandparents
Shrinking Violet: Violet. Hey...
Snap Back: Between the last book written by Warner, and the first by the publishing company, the Alden children's ages were reset to 14, 12, 10, and 6.
Status Quo Is God: Have you picked up what the most memorable feature of this series is yet? You can read any book past #19 in any order you like.
Team Pet: Watch the dog, who technically belongs to Jessie.
Twofer Token Minority: The Boxcar Kids get a female Korean adopted cousin.