Hello Milind
Children travel absolutely for free until one day before their 6th birthday.
If you want to make seat reservations for panoramic trains, such as the Glacier Express, you have to pay the seat reservations even for children under 6 years of age, if you want them to have their own seat.
More information on travelling with children is given here:
https://www.sbb.ch/en/travel-information/individual-needs/travelling-with-children.html
Kind regards,
Roland