Hello TrainNovice206
Sorry for the delayed reply.
Will the round trip include an overnight at Zermatt? It would be quite a long trip if all is going to happen in one day.
I have a hard time to give you a reasonable advice on the ticket question as I do not know for how long you will stay in Switzerland and what your other trips will include.
For a simple return trip (Geneva-Zermatt-Geneva), I would say, buy the Saver Day Pass for each adult/day, and the Junior Card for each child. The latter costs CHF 30 per child, is valid one year, and the child travels for free while accompanied by at least one parent with a valid ticket. For tourists, the Junior Card is available at the SBB travel office only. The Junior Card is also recognised by some of the lift operators in Zermatt (check with them on-site).
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
If you stay longer in Switzerland / Zermatt, in addition to the tickets, the Swiss Half Card would be very beneficial for the parents. It would also come with the complimentary Swiss Family Card which is the equivalent of the Junior Card yet free of charge.
https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-halffare-card.html
https://www.myswitzerland.com/en-ch/planning/transport-accommodation/tickets-public-transport/swiss-family-card/
As you are mentioning the Matterhorn Alpine crossing: beware that this mountain experience is currently closed due to unplanned maintenance. As I recently read in a newspaper, the operator expects the reopening by end of March 2024 but is not yet confirmed. By all means, avoid purchasing mountain excursion tickets in advance if you want to avoid the often cumbersome ticket refund process.
Please let us know if you have got more queries. We might be able to give you a more distinct advice once we know all details about your travel plans.
Kind regards,
Roland