Hi ScotlandTravels
Discounted (Half-Fare) tickets are only valid with a Half-fare (Halbtax) card. So I don’t suggest you buying discounted tickets for your parents. We do not have senior discounts in Switzerland for public transport.
A Half-fare card is only recommended if they are going to do trips which will cost more than 240 CHF per person.
Hope that helps a bit, best regards
Zebis