Swiss Half Fare Pass from Milan into Interlaken ?

We intend to buy a Swiss Half fare pass whilst visiting Interlaken & Lucern surroundings. We will be coming into Interlaken by rail from Milano Centrale. How can we stay on board the train bound for Interlaken with the ticket purchased via Half Fare Pass rate ? We have the following options . Could you advice us which one is the best and most realistic option. Thank you .

Option A:
We intend to buy our normal ticket from Milano Centrale to Domodossola inadvance from Singapore via Trenitalia website. Is it possible to buy the other part of our journey from SBB.com web site (with Half Fare Pass ) from Domodossola to Interlaken ? Can we stay on board the train without the need to validate the Half Fare Pass when the train arrived at Domodossola station ?

Option B:
Or is it possible to by all the way from Milano Centrale to Interlaken from Trenitalia website from Singapore. Is there a field in Trenitalia website that allows me to select Half Fare Card holders for reduced fare into Interlaken ?

Thank you everyone for your kind assistance.

Dear Arno, Thanks for your reply. We have now booked in Trentalia site : Milano Centrale >> Domodossola for Euro71.40 in order to connet to the next train from Spiez departing at 14.05 hrs. We will book in SBB site our next leg from Domodossola >> Interlaken Ost 30 days before our departure date of 15 June. One question, I did a test booking in SBB site for Domodossola >> Interlaken . Under Adult I can select Half Fare Card at CHF26.50. However, when it comes to the children, now there is no Junior Card / Family Card for me to select?? My kids are 9 yrs and 13 yrs. We have bought the Half Fare Cards and requested for Family Card . Can we we just make booking for 3 adults since the next change will only be in Spiez ? Is it the norm with a Family Card, the children need not have a ticket all the way to Interlaken or any train they accompany their parent in ? Thanks for your reply.

Hello,

The children indeed do not need an additional ticket if you have the Swiss Family Card.