Please try to change the date & time of your device to be Swiss local time. This might help.
For children, you do not need to buy a Saver Day Pass. A Children Day Pass at CHF 19 is sufficient and equals the Saver Day Pass in terms of range of validity. Or, you could buy the Junior Card at the SBB Travel Center once you are on-site; it costs CHF 30 per child, is valid for 1 year and your children travel for free when accompanied by you while you hold a valid ticket such as the Saver Day Pass.
Day Pass Children | SBB
When you plan multiple journeys using public transport, you should consider the Swiss Travel Pass, the Swiss Half Fare Card, or any regional pass such as Tell Pass or Berner Oberland Pass.
Swiss Travel Pass – the All-in-one-Ticket for Switzerland | SBB
The Half Fare Travelcard on the SwissPass | SBB
Ps. Kindly see our English-speaking SBB Community. There, this topic has been treated already: Cannot Purchase International Ticket from Australia