The Little House book series is considered to be appropriate for children in grades 3-7, although it may also be read and enjoyed by children outside of this range based on their reading skill level. The books are written at a level that is appropriate for upper elementary and middle school-aged children.
They contain complex themes and ideas, but the language and sentence structures used are simple enough to be understood by children in these grade levels. The books are also used as a teaching material in classrooms and are often used as a part of reading and social studies curriculum.
The later books in the series do get more advanced with experiences and vocabulary, but many readers tend to read the series through in its entirety.