The Magic School Bus in the Rain Forest is an adaptation of the Magic School Bus television episode "In the Rain Forest"... sort of. While much of the plot and even some of the dialogue is lifted straight out of that episode, the focus of the story and some key plot elements are changed.
The changes (such as who bought the cocoa tree for Ms. Frizzle and why, Inspector 47's plan to keep out the peccaries, and the general state of the rain forest itself) serve to shift the story's focus; in the episode version, the lesson's focus was the web of life and how the plants and animals in an ecosystem are all interconnected, while the picture book's lessons is on the rain forest itself.
Though fans of the television show--such as myself--may be vaguely annoyed to see an episode's plot appropriated to teach a different lesson, The Magic School Bus in the Rain Forest is a good way to teach a young child about rain forests. I would, however, definitely recommend complementing this with a viewing of the episode.