Hello Necip_Erdogdu
Children pay in Switzerland from the age of six, but as you are travelling from abroad, both children need a ticket to the border if they want a seat..
After that, the 3 year old can travel for free and the 7 year old with the Swiss Family Card if you buy the Swiss Travel Pass. If you are only travelling this route, I recommend buying a normal ticket.
Have a nice day. Greetings, AuroraD