The Saver Day Pass is fine as standalone ticket and does need the GA Travel Card in addition. You can simply get on board the train, bus, etc.
In terms of validity, the Saver Day Pass does have the same area of validity as the GA Travel Card, that’s why the GA is mentioned; you can find the map of validity here, in section Downloads.
Children up to 5.99 years of age do not need a ticket.
Arriving 10 minutes before departure is absolutely sufficient; trains often arrive a few minutes before departure only. You will see, taking trains in Switzerland is just as taking suburban trains in cities, it’s mass transit & simple to use
Let us know should you have more questions.