force-sub-categories-use-the-parent-category-template from http://werdswords.com/force-sub-categories-use-the-parent-category-template/

force-sub-categories-use-the-parent-category-template.php

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' );