In addition to @FatimaH ’s great advice, I wish to add that you can get the Junior Card or Children Co-Travel Card at CHF 30 per child, valid for one year. With it, the child (6-15.99 years of age) travels for free when accompanied by you, and you hold a valid ticket. In the contrary to citizens, for tourists, the card is issued as a paper ticket. You can get it from any SBB Travel Center at the stations.
Alternatively, there is the Children Day Pass at CHF 19, valid for the entire public transport for 1 day (restrictions on mountain railways do apply). Whenever the ticket price for a half price ticket is more than CHF 19, simply buy the Children Day Pass - it is a great bargain.
Day Pass Children | SBB
You can find a wealth of information on travelling with children in Switzerland on this website; there are all the ticket options mentioned:
Tickets and travelcards for children and young people | SBB