Custom MySQL function times wrong, [ERR] 1418-this function has none of deterministic ...

There was an error today when I executed the SQL for my custom MySQL function , sql as follows:

/**
Custom MySQL function getchildlist
*/
Delimiter//
CREATE FUNCTION ' Pengwifi_wifi '. ' Getchildlist ' (Rootid INT)
RETURNS varchar (1000)

BEGIN
DECLARE stemp VARCHAR (1000);
DECLARE stempchd VARCHAR (1000);

SET stemp = ' $ ';
SET stempchd =cast (Rootid as CHAR);

While stempchd are NOT NULL does
SET stemp = concat (stemp, ', ', stempchd);
SELECT Group_concat (ID) into Stempchd from Hui_class where Find_in_set (parent_id,stempchd) >0;
END while;
RETURN stemp;
END
//

Error message: [ERR] 1418-this function has none of the deterministic, NO SQL, or READS SQL DATA in its declaration and binary logging IS-enabled (you *might* want-use the less safe log_bin_trust_function_creators variable)

WORKAROUND:set global log_bin_trust_function_creators=true;

Custom MySQL function times wrong, [ERR] 1418-this function has none of deterministic ...