Hello Whaimiphoa35
The Swiss Family Card is their ticket. Children travel for free while holding the Swiss Family Card and being accompanied by their parents who do hold themselves a valid ticket.
You will only need a seat reservation for your kids when travelling on certain PE trains, for example the Glacier Express, where a seat reservation is required for all passengers. In that case, they travel again for free but you need a seat reservation for them. On regular trains (IC, IR, RE, etc.), you do not need seat reservations as there is free seating.
Same for you, if you hold the Swiss Travel Pass, you do not need to book tickets. You can simply board any train, the Swiss Travel Pass is your ticket.
Does that make sense?
Kind regards,
Roland