The best episode in this book is that an elephant swung the enormous crocodile round in the air, and then the crocodile was launched into space as a green rocket. Do you want to know where the the rocket arrived in the end and what happened to him there? この表現は自然ですか？

"The best part of this book (maybe you were trying to say, the best chapter in this book? In that case you would say "the best chapter in this book was the one where") was when the elephant swung an enormous crocodile around in the air, and then launched the crocodile into space, so that he looked like a green rocket! Do you know where the crocodile arrived in the end and what happened to him there?"
Pretty good for the most part! Just a few mistakes! :D