remove the parent categories from categories widget

function exclude_parent_categories( $args ) {
    $args['exclude'] = get_parent_categories();
    return $args;
}

function get_parent_categories() {
    $parents    = array();
    $categories = get_categories( array( 'hide_empty' => false ) );
    foreach ( $categories as $category ) {
        if ( $category->parent !== 0 ) {
            $parents[] = $category->parent;
        }
    }
    return implode( ',', $parents );
}

add_filter( 'widget_categories_args', 'exclude_parent_categories' );