The Railway Chidlren

Author:  E. Nesbit
Lexile measure:  940, but many adaptations with lower Lexiles
Bottom line:  Good book, but mature topics may make this unsuitable for independent reading for many young children.
Language:  No swear words, no potty language, some use of dialect.
Violence:  One of the adventures involves a burning barge, another involves a boy who falls and breaks a leg, and another where the boy gets hurt with a rake during a sibling squabble.
Big Kid Mature Topics:   The father in the story (warning: spoiler) is taken away from the home to prison because of a crime he didn't commit.  The mother gets a serious case of influenza.  The children live in poverty and often ask for others to help them or to help others.  In one chapter, the boy pretends to be mining coal, but is really stealing it from the railroad yard.  He is caught but is forgiven.
Plot: moderately complex.

This is a nice book and I can see why it is used frequently as a textbook.  There is definitely a lot of room for discussion surrounding the morality of the actions of the children and the building of suspense.   In these troubled economic times, this book might hit very close to home for some children.  It might be too close for comfort for some.  If your family is under significant financial strain at this time, you might consider reading something else or using this as a read aloud instead of for independent reading.