You can change how subcategories are displayed in the settings of the parent category.
In the settings section under Appearance, you can tick “Show subcategory list above topics in this category” and choose “boxes” in the dropdown below. (Ignore the “default list filter” in the screenshot, I just reused it.)
There is also a topic about the Available settings within the category edit page and a guide especially about how to Display boxes for subcategories above topic list.