CREATE OR REPLACE PROCEDURE treeDown(INTEGER)
LANGUAGE plpgsql
AS $$
DECLARE
    cur_childs CURSOR FOR select * from TreeRelation where von = $1 AND from < to;
    rec RECORD;
BEGIN
    FOR rec IN cur_childs LOOP
        CALL treeDown(rec.to); -- seems to die here
        RAISE NOTICE '% is a child from %', rec.to, $1;
    end loop;
END;
$$;

...
FOR rec IN (SELECT *
                   FROM treerelation
                   WHERE ...) LOOP
...

-- 3 is our start node in this case
WITH RECURSIVE test(s) AS (
            select to from TreeRelation where from = 3 AND from < to
        UNION
            select to from test, TreeRelation where from = test.s AND from < to
        ) select * from test;