Use parent category template for subcategories (see http://werdswords.com/force-sub-categories-use-the-parent-category-template/)

wp-subcats-use-parent-cats-template.php

<?php
/*
 * Use Parent Category
 * Description: Causes subcategories to use their parent's template
*/
function new_subcategory_hierarchy() {
    $category = get_queried_object();

    $parent_id = $category->category_parent;

    $templates = array();

    if ( $parent_id == 0 ) {
        // Use default values from get_category_template()
        $templates[] = "category-{$category->slug}.php";
        $templates[] = "category-{$category->term_id}.php";
        $templates[] = 'category.php';
    } else {
        // Create replacement $templates array
        $parent = get_category( $parent_id );

        // Current first
        $templates[] = "category-{$category->slug}.php";
        $templates[] = "category-{$category->term_id}.php";

        // Parent second
        $templates[] = "category-{$parent->slug}.php";
        $templates[] = "category-{$parent->term_id}.php";
        $templates[] = 'category.php';
    }
    return locate_template( $templates );
}

add_filter( 'category_template', 'new_subcategory_hierarchy' );