Hello Dibyendu_INDIA
You can buy all tickets in the SBB app or on sbb.ch. Indicate « Half Fare Travelcard » as discount for the adults. No check is made if you really have a discount when you buy the tickets, you must show the tickets and the Half Fare Cards in the train with your physical passports.
I suggest to create an account and not to buy tickets as guest, but you may have already created one to purchase the cards.
If you also defined the name of your child when you bought the Half Fare Cards, you have a Family Card and she/he doesn’t need any other ticket.
Kind regards
Danhiel
PS Sorry, I didn’t see that RolandP had already answered your question.