Try this:
SELECT t.Sno, t.childid, c.customername AS NAME, t.earnedmoney AS commission,
p.bookingamt AS bookingamt, p.totalarea AS totalarea,
MAX(STUFF(A.childid, 1, 1, '')) AS childs
FROM tbl_level t
LEFT JOIN customerprofile c ON c.cusid = t.childid
LEFT JOIN propertyregistration p ON p.applicationno = c.applicationno
CROSS APPLY(SELECT ' ' + t1.childid FROM tbl_level t1 WHERE t1.parentid = t.childid FOR XML PATH('')) AS A (childid)
WHERE parentid = @id
GROUP BY t.Sno, t.childid, c.customername, t.earnedmoney, p.bookingamt, p.totalarea
To fetch customer name from all child ids:
SELECT t.Sno, t.childid, c.customername AS NAME, t.earnedmoney AS commission,
p.bookingamt AS bookingamt, p.totalarea AS totalarea,
MAX(STUFF(A.customerNames, 1, 1, '')) AS childs
FROM tbl_level t
LEFT JOIN customerprofile c ON c.cusid = t.childid
LEFT JOIN propertyregistration p ON p.applicationno = c.applicationno
CROSS APPLY(SELECT ',' + c1.customername
FROM tbl_level t1
INNER JOIN customerprofile c1 ON c1.cusid = t1.childid
WHERE t1.parentid = t.childid
FOR XML PATH('')
) AS A (customerNames)
WHERE parentid = @id
GROUP BY t.Sno, t.childid, c.customername, t.earnedmoney, p.bookingamt, p.totalarea