Hello jmondru
Yes, you should be able at the RhB station.
https://www.rhb.ch/en/information/stations/tirano/
If not, purchase two Children Day Passes and ask the conductor on the train to confirm the tickets. Then go to a station later to get the Swiss Family Card and the (partial) refund of the Children Day Passes.
Btw, Swiss people love niceties. Saying hello and please when asking a question or when asking for help will be a great motivation to them to indeed offer help …
Kind regards,
Roland