Exclude Subcategories by Parent id/name

I need to exclude all the subcategories + the parent category by only giving the parent category id/name, (in the functions.php file). This is I’ve tried so far:

function fb_filter_child_cats($query) {

        $cat = get_term_by('name', $query->query_vars['category_name'], 'category');
        $child_cats = (array) get_term_children( &$cat->term_id, 'category' );

        if ( !$query->is_admin )
            $query->set( 'category__not_in', array_merge($child_cats) );

        return $query;
    }
    add_filter( 'pre_get_posts', 'fb_filter_child_cats' );

and this:

function exclude_category($query) {
	$child_cats = (array) get_term_children('20', 'category');
	$child_cats = (array) get_categories('child_of=10')

	if ( $query->is_home ) {
		$query->set('category__not_in',array_merge(array('20'), $child_cats));
	return $query;
	}
}

add_filter('pre_get_posts', 'exclude_category');

and this:

if ( !function_exists('fb_filter_child_cats') ) {
    function fb_filter_child_cats( $cats ) {
        global $wp_query, $wpdb;

        if ( is_category() ) {

            // get children ID's
            if ( $excludes = get_categories( "child_of=" . $wp_query->get('cat') ) ) {

                // set array with ID's
                foreach ( $excludes as $key => $value ) {
                    $exclude[] = $value->cat_ID;
                }
            }

            // remove child cats
            if ( isset($exclude) && is_array($exclude) ) {
                $cats .= " AND " . $wpdb->prefix . "term_taxonomy.term_id NOT IN (" . implode(",", $exclude) . ") ";
            }
        }

        return $cats;
    }

    if ( !is_admin() ) {
        add_filter( 'posts_where', 'fb_filter_child_cats' );
    }
}

but nothing seems to work.