Hello Chrisford30
Thank you for your question.
Children from their 6th up to their 16th birthday accompanied by at least one parent (holding a Swiss Travel System ticket) travel free of charge on train, bus, boat and mountain railway with the complimentary Swiss Family Card.
To get the Swiss Family Card you need to add your children as passengers under ‘Add passengers’ when you buy your Swiss Travel Pass.
If you already purchased the STP and have not added them, please call our colleagues at the SBB Contact Center (0041 848 44 66 88, CHF 0.08/min): after checking with them, purchase a new STP (ncluding kids) while on the line, so that they can see the new order, and then wait for them to refund the first purchase.
This operation should be no problem if the validity of your STP is in the future.
Have a great time in Switzerland!
EleonoraS