Hello Ajay
This is why I always suggest, to first inform yourself on the ticket options before purchasing any tickets.
You can ask for a refund. Exchanging full fare Supersaver tickets against half fare Supersaver tickets is one of the few exceptions where a refund of Supersaver offers is possible at all.
Conditions:
https://sbb-tickets.cc/en/help-and-contact/refunds-compensation/refunds-compensation/ticket-refunds.html#anchor4
Refund form:
https://sbb-tickets.cc/en/help-and-contact/refunds-compensation/refunds-compensation/refund-form.html
You have to purchase the new Supersaver tickets first, to prove, you have purchased them. Same for the Swiss Family Card for your son.
Also, there is a fee of CHF 10 per refund request. You have to check if the money you gain through the refund is higher than the CHF 10; otherwise you can simply travel with the existing tickets.
Supersaver tickets are only valid for one particular connection. If you miss the departure, the tickets are lost. Also, you must carry your physical passport or ID card issued by the government when using e-tickets such as Supersaver tickets.
Kind regards,
Roland