// Use a parent category slug if it exists
function child_force_category_template($template) {
    $cat = get_query_var('cat');
    $category = get_category($cat);

    if ( file_exists(TEMPLATEPATH . '/category-' . $category->cat_ID . '.php') ) {
        $cat_template = TEMPLATEPATH . '/category-' . $category ->cat_ID . '.php';
    } elseif ( file_exists(TEMPLATEPATH . '/category-' . $category->slug . '.php') ) {
        $cat_template = TEMPLATEPATH . '/category-' . $category ->slug . '.php';
    } elseif ( file_exists(TEMPLATEPATH . '/category-' . $category->category_parent . '.php') ) {
        $cat_template = TEMPLATEPATH . '/category-' . $category->category_parent . '.php';
    } else {
        // Get Parent Slug
        $cat_parent = get_category($category->category_parent);

        if ( file_exists(TEMPLATEPATH . '/category-' . $cat_parent->slug . '.php') ) {
            $cat_template = TEMPLATEPATH . '/category-' . $cat_parent->slug . '.php';
        } else {
            $cat_template = $template;
        }

    }

    return $cat_template;
}
add_action('category_template', 'child_force_category_template');