If you mean like this:
Go to the parent / main category page and select the wrench icon to allow you to edit the category.
Click on “settings” and scroll down until you see this area:
“show subcategory list above topics in this category”
Select that option, and choose “boxes” as the dsiplay style.
If you mean on the /categories page, this is not an option. The only option there would be to edit the CSS to unhide the subcategories in order to show them inside the parent’s boxes.