Add missing rows to table via checking relations between other tables
INSERT INTO product_m_tender (tender_id, product_fix_id)
SELECT tender_id, pairs.child_product_id
FROM product_m_tender
JOIN (SELECT parent_product_id, child_product_id FROM products_m_products
      UNION
      SELECT child_product_id, parent_product_id FROM products_m_products) pairs
  ON product_m_tender.product_fix_id = pairs.parent_product_id
ON CONFLICT (tender_id, product_fix_id) DO NOTHING;


fiddle (nextvals absence fixed by a constant of zero).

Pay attention that 

CONSTRAINT tender_id_product_fix_id UNIQUE (tender_id, product_fix_id) 


is added to product_m_tender table.

If possible products chain can be longer than 2 records, than execute this query until zero records affected, or move pairs subquery to recursive CTE.