hi Sonja.
don't quote me on it, but i believe i read that Juliet Laroche put her two young daughters on the bottom bunk and put chairs against the side to form a makeshift cot.
White Star's policy was one berth per full ticket price. So children travelling at half price were expected to share if the ship was fully booked. And certainly they didn't provide a separate berth for an infant travelling free of charge.