bbp_get_all_child_ids( int $parent_id, string $post_type = 'post' )

function bbp_get_all_child_ids( $parent_id = 0, $post_type = 'post' ) {
	global $wpdb;

	// Bail if nothing passed
	if ( empty( $parent_id ) )
		return false;

	// The ID of the cached query
	$cache_id  = 'bbp_parent_all_' . $parent_id . '_type_' . $post_type . '_child_ids';

	// Check for cache and set if needed
	$child_ids = wp_cache_get( $cache_id, 'bbpress_posts' );
	if ( false === $child_ids ) {

		// Join post statuses to specifically exclude together
		$not_in      = array( 'draft', 'future' );
		$post_status = "'" . implode( "', '", $not_in ) . "'";

		$child_ids   = $wpdb->get_col( $wpdb->prepare( "SELECT ID FROM {$wpdb->posts} WHERE post_parent = %d AND post_status NOT IN ( {$post_status} ) AND post_type = '%s' ORDER BY ID DESC;", $parent_id, $post_type ) );
		wp_cache_set( $cache_id, $child_ids, 'bbpress_posts' );
	}

	// Filter and return
	return apply_filters( 'bbp_get_all_child_ids', $child_ids, (int) $parent_id, $post_type );
}