There is a difference between the books originally done by Joanna Cole and the books that are companions to the PBS Television series. Joanna Cole's books tend to be more in depth and more appropriate for older children, whereas the books that go with the TV series are pretty easily understandable for younger children (5+?) There are also beginner chapter books that go with the series which again are more appropriate for older children (7+?). However, that said, my toddler has absolutely loved both the Joanna Cole books and the TV series picture books, as long as we are able to answer all of his questions. A wonderful feature of the books, is a brief blurb at the end of the book that sort of differentiates between what is real and what is pretend--for example by discussing how things can't really shrink, or how it would actually take years for the Magic School Bus to go to Mars, etc. These sections also often mention that there is more to a topic than what has been discussed (ie there are many different species of bats) and that can be used as a launching point for more exploration.