Children under 6 years of age.
Children under 6 years travel free of charge when accompanied by a person holding a valid ticket. The accompanying person must be at least 12 years old and may then take up to 4 children with them free of charge. From 16 years of age, up to 8 children may travel with them. Half-price tickets must be purchased for each additional child, or any designated minimum fare must be paid.

Copy from SBB website.