Discussing a book with a child is one of the best ways to model how to think like a geographer. I teach 4th grade. That is an excellent question. And there is no right answer. The goal is to understand the concept of region.
Geographers define a region as an area that is similar in some way.
The similarity can be based on some uniform characteristic, like having the same landform, or the same climate, or it can be similar based on a connection, like an area that is part of a river system, or part of the distribution area of a newspaper. Regions are a tool we use to help make sense of complicated patterns. There are other types of regions as well but they don't necessarily fit the geographic definition. We commonly divide up the world into convenient chunks to be able to talk about it. We talk about the Midwest, the South, and the West, for example, or Western Europe, and Sub-Saharan Africa.