Same template for Category and Subcategory

Using the same template file for a category and its children is not possible with Wordpress. In order to do you need to use Elevate Parent Category Template-Plugin. If you using the Plugin you'll be able to use some adittional template functions:

• get_category_child()
  returns the current post child category ID.
• is_parent()
  checks if the current post is at the parent category.
• get_category_title($id)
  gets the title of the category that is $id.
• get_category_parent()
  returns the current post's parent category ID.