Gertrude Chandler Warner's The Boxcar Children (1942) was one of my favorites when I was very young, so I was not surprised to find out that Ms. Warner taught first grade for a very long time. The book is very easy to read, but is not as short as emergent reader books tend to be today, which is probably one of the reason I liked it!
That, and what child wouldn't like the story of four children surviving on their own? They live in a boxcar, find dishes and furnishings at the dump, and the older brother mows yards for people to get them money for lots of bread and milk. When this was first published, there were complaints that the children were enjoying themselves too much without adult supervision.
Surprise Island and The Yellow House Mystery are the second and third books in the series. Ms. Warner wrote 19, and then The Albert Whitman Company continued the series for a very long time. While the mysteries are okay, they lose a lot of their charm for me because the grandfather, who adopts the children, makes everything immediately possible, even though there is still a lot of time that the children are on their own. (Really? Sending a 16,14, 12 and 7 year olds to an island with just a caretaker to watch them?) They are also still really , really interested in dishes, and still eat a lot of bread and milk.
These are books that just are not the same to read when one is not about eight years old. Sigh.