Using STUFF to get children value grouped by parent ID

Question

I'm looking for a solution to get children's values in a single cell, I've found out about STUFF but when I'm trying to apply it to a self-referencing table it does not work.

Neither CONCAT nor CONCAT_WS work 'not a recognized built-in function name'.

Table:

ID	ParentID	Value
1	1	HELLO
2	1	Foo
3	1	Bar
4	3	Blob

My query at this point :

SELECT TaskParent.TaskID,
    ValueValues = STUFF((
            SELECT DISTINCT ' | ' + TaskChildren.Value
            FROM tblTask TaskChildren
                JOIN tblTask TaskParent2 ON TaskParent.TaskID = TaskChildren.TaskGroupID
            WHERE
                TaskParent2.TaskID = TaskParent.TaskID
                AND TaskParent2.TaskID not like TaskParent2.TaskParentID
            FOR XML PATH('')
            ), 1, 3, ''
        )
FROM tblTask Task
    JOIN tblTask TaskParent ON TaskParent.TaskID = Task.ParentID
GROUP BY TaskParent.TaskID

In the result from the query above I'm getting :

ParentID	Value
1	Foo

But I want :

ParentID	Value
1	Foo / Bar

I'm sorry if this was already answered, but I already spent too much time looking for it.

For more details when I add Task.ID to the query the result is the following :

ID	ParentID	Value
2	1	Foo
3	1	Bar

Here is modifications I made on @@siggemannen solution

select TaskParents.ID, substring((select ' | ' + Value from tblTask TaskValues where TaskValues.ParentID = TaskParents.ID for xml path('')), 4, 99999) as Children
from tblTask Task
    LEFT JOIN tblTask TaskParents ON TaskParents.ID = Task.ParentID
group by TaskParents.ID
ORDER BY TaskParents.ID

Answer 1

Something like this perhaps:

select *
into tblTask
from (
    VALUES  (1, 1, N'HELLO')
    ,   (2, 1, N'Foo')
    ,   (3, 1, N'Bar')
    ,   (4, 3, N'Blob')
) t (ID,ParentID,Value)


select ID, substring((select ' | ' + value from tbltask t2 where t2.parentid = t.id for xml path(''), type).value('.', 'nvarchar(max)'), 4, 99999) as Children
from tblTask t
group by ID

Edit: Alternative method without TYPE:

select ID, substring((select ' | ' + value from tbltask t2 where t2.parentid = t.id for xml path('')), 4, 99999) as Children
from tblTask t
group by ID

This returns whatever children a parent has:

ID	Children
1	HELLO | Foo | Bar
2	NULL
3	Blob
4	NULL