Hello Bothoazouf16
Unfortunately not.
The Junior Card for tourists is only issued in paper form at any SBB Travel Center in Switzerland. You would have to get it in Brig, when transferring there to the train to Zermatt.
Do the parents plan to get the Swiss Half Fare Card? If yes, the Swiss Family Card (the equivalent to the Junior Card) is complimentary when buying the Swiss Half Fare Card or the Swiss Travel Pass. It would be available as online ticket.
Kind regards,
Roland