Hello sriar
If you intend to purchase the Swiss Half Fare Card, make sure you do request the complimentary Swiss Family Card. Only with the Family Card, your son will travel for free when accompanied by a parent holding a valid ticket. Your son does not need a ticket in addition, you do not have to mention him while purchasing tickets.
Provided you do not have the Swiss Half Fare Card and the Swiss Family Card, you could purchase for your son the Day Pass Children. In terms of validity, it is the same ticket as a Saver Day Pass yet simply for children.
Please let us know should you have more queries.
Kind regards,
Roland