3岁儿童买SBB车票

从苏黎世到慕尼黑。想给小朋友自己一个座位，不抱他。请问怎么选？
因为准备房车自驾，是不是也没有必要购买SWISS PASS？还是也买SP划算？（路线上德国就慕尼黑和斯图加特两个城市，主要在瑞士境内开）


* Child without their own seat on daytime trains
*The parents of the child/young person are travelling with them
*The grandmother/grandfather of the child/young person is travelling with them