Hello Arnie Z
You do not need to reserve seats. Swiss trains are mass transit and there is free seating. Holding a valid ticket, you simply get on board and take any vacate seats. Outside of rush hours, there are plenty of seats left. And, optional seat reservations are only possible for IC, ICE, RJX, and TGV trains.
Kindly be informed that you must activate the travel day no later than before getting on the train; a hefty surcharge is applicable if you fail to do so.
From my point of view, the Eurail Pass is not very convenient for Switzerland as it only covers the main lines, but neither buses nor mountain railways.
The Swiss Travel Pass or the Swiss Half Fare Card, both do come with the complimentary Swiss Family Card offering free travel for kids (6-15.99), usually mean the better deal, especially when you plan to go on mountain excursions.