One way to hold it: the two words mean exactly what they say — a seat, inside a car. Picture opening the door and looking at the thing you sit on. Everything this page covers is that object. The only question a listener has to settle is which seat: any of the ones built into the car, or the small extra one that is on top for a child.
From the picture
另外裝上去、給小孩坐的那一張
同一張椅子的畫面縮小了範圍：不是車上本來的位子，而是另外買來、綁在位子上面的那一張小椅子。這是日常用法把一個字面組合收窄成專指某樣東西，不是另一條歷史來源。
noun · B1
A special seat for a baby or young child that is fixed onto a seat in a car with straps or a belt, so the child rides safely and sits at the right height.
兒童安全座椅
noun · A2
A seat inside a car for a person to sit on — the driver's seat or any of the passenger seats, including the back ones.
車上的座位