Alternatively, you can use this. I'm assuming that the column PHOTO is in the USER table. I see that some of the columns in the query have not been prefixed with an alias, it is always a better practise to prefix all column names with alias, it improves performance too...
SELECT
K.KIDID,
K.STUDENTNUMBER,
KIDNAME,
KIDLASTNAME ,
SC.SCHOOLNAME ,
KIDGRADE ,
KIDCLASS ,
SC.SCHOOLID,
k.TEMP_BARCODE,
(SELECT PHOTO FROM USER UP WHERE U.USERID = UP.USERID) AS PHOTO,
A.ACCOUNTID
,SUM(TRANSACTION_AMOUNT) AS [BALANCE]
FROM
KIDS_DETAILS K
INNER JOIN SCHOOL SC
ON K.SCHOOLID = SC.SCHOOLID
INNER JOIN PARENTKID PK
ON PK.KIDID = K.KIDID
INNER JOIN USERS U
ON U.USERID = K.USERID
INNER JOIN ACCOUNTS A
ON A.USERID = U.USERID
INNER JOIN ACCOUNTRANSATIONS AT
ON AT.ACCOUNTID = A.ACCOUNTID
LEFT OUTER JOIN KIDSPHOTO KP
ON KP.KIDID = K.KIDID
GROUP BY
K.KIDID,
K.STUDENTNUMBER,
KIDNAME,
KIDLASTNAME ,
SC.SCHOOLNAME ,
KIDGRADE ,
KIDCLASS ,
SC.SCHOOLID,
k.TEMP_BARCODE,
A.ACCOUNTID