Create table #t (id_parent int not null, id_child int not null, primary key clustered (id_parent, id_child))

insert into #t (id_parent, id_child)
values
(1, 2),
(2, 2),
(2, 3),
(2, 4),
(4, 3),
(3, 1),
(5, 5),
(5, 6),
(5, 7),
(7, 5),
(8, 8)

WITH cte AS
(
    SELECT id_parent,
           id_child, 
           CAST('/' + CAST(id_parent AS varchar(max)) + '/' AS varchar(MAX)) AS path
    FROM #t
    WHERE id_parent = 1
    UNION ALL
    SELECT t.id_parent, 
           t.id_child, 
           CAST(cte.path + CAST(t.id_parent AS varchar(MAX)) + '/' AS varchar(max))
    FROM   cte
            INNER JOIN #t t ON cte.id_child = t.id_parent
    WHERE   cte.path NOT LIKE '%/' + CAST(t.id_parent AS varchar(MAX)) + '/%'
)
SELECT id_parent, cte.id_child
FROM cte