I apologise for the belated response. I just found your question in the list of open queries.
As a general saying, children aged 6 to 15.99 also need a ticket. This can be a half fare point-to-point ticket or a Children day pass, what ever is less costly.
When they travel with you, however, you could get the Junior Card from the counter for them. It costs CHF 30 for 1 year, and it will grant free travel whenever the child travels with you while you hold a valid ticket for yourself. For tourists, the Junior travel card is issued as paper ticket.
The 3 year old does not need a ticket at all.