Hello Ghickoickoa96
You probably missed to declare your children when you purchased the passes from Agoda. I do not know this platform at all.
Why cant I get Swiss Family Card by showing my Adult Passes at some train station ? is that not possible ?
It certainly is, when you have purchased the passes from SBB directly yet missed to declare the children. Whether SBB is able to do so for a complimentary product that you have got from a reseller, I do not know; you would have to ask SBB:
https://sbb-tickets.cc/en/help-and-contact/customer-service/praise-comments/reklamation.html?stepId=3.17
Usually, SBB cannot amend tickets whose booking is not on their file; this is the same for air travel tickets: if you had booked the flight through a travel agency, the airline will not amend your ticket but send you back to the travel agency.
Kind regards,
Roland