Based on what you have and the output that you have shown, the following should give you what you're looking for...

SELECT

m1.ParentID AS Account_ID,

STUFF((

SELECT ', ' + CAST(m2.ChildID AS VARCHAR(8))

FROM #mytable m2

WHERE m1.ParentID = m2.ParentID

FOR XML PATH('')), 1, 2, '') AS ChildList

FROM

#mytable m1

GROUP BY

m1.ParentID

Here is the output...

Account_ID ChildList

----------- ------------

1 1, 2, 3

2 3, 5

5 1

HTH,

Jason