Hello, it looks as though it is a series title. Wikipedia has a article on the series. The original books were written by one Gertrude Chandler Warner, but other authors continued the series, which now has approximately 100 books. I hope your grandchildren like reading a LOT.
Quote:
The Boxcar Children is a children's literary franchise originally created and written by American writer and first-grade school teacher[1] Gertrude Chandler Warner and which today includes well over 100 titles. The series is aimed at middle readers in grades 2–6.
Originally published in 1924 by Rand McNally and reisssued in 1942, the novel The Boxcar Children tells the story of four orphaned children, Henry, Jessie, Violet and Benny, who create a home for themselves in an abandoned boxcar. They fear their legal guardian, their grandfather, believing him to be cruel. They enjoy their freedom but find their lifestyle has many problems and is not a long term solution. They eventually meet their grandfather, James Alden, who is a kind and wealthy man. The children agree to live with him. James moves the beloved boxcar to his backyard, so the children can use it as a playhouse. In the subsequent books, the children encounter many adventures and mysteries in their neighborhood or at the locations they visit with James. The majority of the books are set in locations the children are visiting over school holidays.
Only the first 19 stories were written by creator Warner. Other books in the series have been created by other writers, but always feature the byline "Created by Gertrude Chandler Warner". The recent books in the series are set in the present day, whereas the original books were set in the 1940s and 1950s.
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More at this link.
The Boxcar Children - Wikipedia, the free encyclopedia
I don't know how many of the books will still be in print, but if they are not, you could find some on a good online bookseller like
Abe Books. I have just looked there and it claims to find 7413 results for Gertrude Chandler Warner. Have fun!