Seat reservations are not mandatory.
On Swiss trains, there is usually free seating. They are a form of mass transit, similar to suburban trains in other countries. As a result, you are welcome to take any vacant seat in your respective travel class. If you want to sit next to someone you don’t know, it’s courteous to ask if the seat is available before sitting down.
You will see once you are here how easy it is to use the Swiss public transport.
Should you reserve a seat? Except for certain panoramic trains (PE), seat reservations on Swiss trains are completely optional and not very common. Swiss people usually make seat reservations when they would like to eat in the dining car, when they travel as a group of 10 or more, or when many travelers are expected. If you still want to reserve your seat, kindly note that this is possible for IC, ICE, EC, IR, RJX, and TGV connections only: Seat reservations in trains and dining cars | SBB
A note concerning your kids: if they are between 6 and 15.99 years of age, you must have the complimentary Swiss Family Card for each of them. They travel for free but you still need to present the Swiss Family Card upon ticket check. Children below 6 years do not need any sort of ticket.