Load different template for sub category from http://www.mindfiresolutions.com/How-to-use-different-template-for-subcategories-in-Wordpress-2277.php

Load different template for sub category.php

/**
 *  Load different template for sub category
 */
function sub_category_template() { 
    
    // Get the category id from global query variables
    $cat = get_query_var('cat');

    if(!empty($cat)) {    
        
        // Get the detailed category object
        $category = get_category($cat);

        // Check if it is sub-category and having a parent, also check if the template file exists
        if( ($category->parent != '0') && (file_exists(TEMPLATEPATH . '/sub-category-template.php')) ) { 
            
            // Include the template for sub-catgeory
            include(TEMPLATEPATH . '/sub-category-template.php');
            exit;
        }
        return;
    }
    return;

}
add_action('template_redirect', 'sub_category_template');