The Magic Tree House Series
In this series, 8 year old Jack and his 7 year old sister Annie travel to a different time period in each installment. The series begins with Dinosaurs Before Dark in which Jack and Annie find a tree house that has mysteriously appeared near their home in Frog Creek, Pennsylvania. In the tree are lots of books. In one particular book, Jack and Annie see scenes of various places and time periods. At the page of the dinosaurs, they say they wish they could go there. Then, presto-magic, the tree house plops down in the middle of dinosaur time.