I would argue that there are two central ideas in Roald Dahl’s delightfully imaginative story. The first idea is the danger of witches, and the second is the importance of family bonds.
The idea of the danger of witches is introduced early in the story, and we learn that our protagonist and unnamed narrator has been taught all about witches by his grandmother. When the story begins, he has already been rescued from witches by his grandmother twice. Grandmamma is deeply concerned about the danger of witches and has told our narrator everything she knows about them, including how to recognize them and that they like to turn children into rodents.
When the boy and his grandmother end up at the same hotel as the Grand High Witch and he overhears that her mission is to turn every child in England into a mouse, he is quickly captured and turned into a mouse himself. His life changes forever due to the witches’ evil powers, making the danger of witches central to the story.
The other central idea that I mentioned is the importance of family bonds. Grandmamma has looked after the little boy ever since his parents were killed in a car accident. A loving, nurturing relationship exists between the boy and his grandmother. She wants to keep him safe at all costs and does not love him any less after he has been transformed into a mouse.