This is a 2nd grade independent reading level chapter book.
Innocent content, adventure with a little suspense. The brother and sister have a little tension with each other as siblings usually do. The language is clean, but these did teach my kids to go around saying "Oh man!" which is funny.
The kids enter a tree house that usually goes back in time or to some other geographic place, and the children are put into danger in some way, but always get out of it just fine.
There are historical facts or science or geography facts thrown in here and there, not enough to justify these are history books.
Both of my boys enjoyed this series.
The general plot line is predictable but kids still love them.
As a read aloud by the parent they get annoying after reading two or three (and they took me 25 minutes to read aloud).
This series is often the first chapter book that a child is willing to read. Some children like to read them over and over while others want to read them just once.
I recommend this series for children who are learning to read and are ready to move up to chapter books.