Hello Ghocaughie72
Our son can travel for free with either parent (or both) without a ticket (…)
Not entirely.
In order for your son to travel for free when accompanied by a parent, you need the Junior Card for him, this in addition to your Half Fare Card. The Junior Card costs CHF 30 per year.
When he travels alone, he needs a half fare ticket, or a Day Pass Children, what ever is cheaper. Or another travel card.
https://www.sbb.ch/en/tickets-offers/travelcards/travelcards-children/junior-travelcard.html
Go to see a counter agent to clarify with them all questions, that would be the most efficient for you.
Does that sound feasible?
For the Saver Day Pass validity, see the map:
https://www.sbb.ch/content/dam/internet/sbb/downloads/en/billette-angebote/abos/Geltungsbereichskarte.pdf.sbbdownload.pdf
Kind regards,
Roland