Hello Vix
Junior cards are always valid if the children are accompanied by at least 1 parent and the parent can show a valid ticket (regardless of whether the ticket is full-fare or half-fare). If you have a Half-Fare travelcard, you can buy the ticket with this discount.
If the children are traveling alone and are under 16 years old, you can buy half-price tickets for them (in the app or at the ticket machine, for example). The children must then show these tickets at the ticket inspection.
All Information are avaiable on our website: https://www.sbb.ch/en/help-and-contact.html
I wish you a nice day.
Kind regards,
FatimaH