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What type of ticket are we talking about?
Children up to 5.99 years of age do not need a ticket, they always travel for free. For children from 6 to 15.99 there is either the Swiss Family Card or the Junior Card. With both, the children do travel for free if accompanied by their parent(s) who hold a valid ticket.
You might ask for a refund using this form. Attach a photo copy of the child’s passport and explain the situation: Refunds and compensation | SBB help & contact
Find more information on travelling with children on this page: Travelling by train with children | SBB