I tried to buy a Junior Travel Card on the SBB website but can't find the page to do so... Does someone know? According to this site (myswissalps.com), I could buy such a card and have it delivered to our home in 3 days.
It can't be purchased separately, only when combined with selected regional passes. I can see how our page confused you, so I added that explanation to the page. You will have to buy it at a staffed train station in Switzerland.
Thanks for asking!