RolandP True, A child having the Junior card does not need a ticket at all when accompanied with a parent.
I do understand the reasoning that the kid might be traveling alone. But it is not, it is traveling with one of the designated parents.
It felt confusing coming from the web app that does pricing correctly.
Another is that if you want to reserve seats, in the web you can do it in one go. while in the SBB app you need to do it in two, adult first: ticket and reservation, and then a 5.- CHF reservation for your kid. And in the mobile phone things are not so easy to navigate always to force you to do two purchases.
Anyway, Thanks for the insights.