4 KudoZ points awarded for this answer. Comment: "Many thanks for the three answers here. My suspicion has been confirmed that there is no single corresponding term. I have used a workaround: for ‘Sportwagenaufsatz’ I used ‘pushchair seat’ and for ‘Kinderwagenaufsatz’ I wrote ‘pram’. For ‘Aufsatz’ on its own I used ‘carriage’ once and ‘pram/pushchair’ on the second occasion. As regards the points here, let peer agrees prevail. And GT’s answer was, as the criteria state, ‘most helpful’."