Hello Gray
No, children up to 5.99 years of age do not need a ticket. You are even allowed to sit your toddler on an unoccupied seat next to you, however during rush hour times it is courteous if you have it seated on your lap. SBB also has special family coaches on certain IC trains. They are designated as “FA” on the platform’s departure board.
For PE trains, for those where a seat reservation is mandatory, you would have to pay for the seat reservation if you wanted to have an extra seat for your kid.
Kind regards,
Roland