It was probable that the babies' skulls were smashed against the unpadded sides of their seats in the accident at Burford, near Oxford, the inquest was told.
Nicholas Gardiner, the Oxford coroner, also called for manufacturers to issue easily understood instructions, after learning that the seats were incorrectly fitted.
Trevor and Yvonne Key, from Nottingham, parents of Christopher and Matthew, are considering legal action against Boots, which sold the seats as its own brand.
Mr Key fitted the pounds 115 Rainbow seats and rear seat belts in the family's Mini Metro. He put them in facing forwards - contrary to instructions - and the belts were incorrectly mounted.
But the coroner, who recorded verdicts of accidental death, said the incorrect fittings would have made no difference. Referring to the seat design, he said: 'There is . . . a very considerable problem.'
Mr Key, 25, told the hearing that in heavy rain he swerved to avoid a lorry on the wrong side of the road and hit another car.Reuse content