Hi Ivy
You can buy the Swiss Travel Pass from here: Swiss Travel Passes | Switzerland Travel Centre
The Swiss Family Card will automatically be added as an option if you enter a passenger below 16 yrs of age. The conditions are the same as for the regular Swiss Travel Pass.
You do not need to purchase another ticket for the Interlaken to Lucerne train. For this route you do not need a seat reservation either. Note, there are construction works on this line until 27 Nov 2022, the line is partially run by buses.
If you allow a personal advice: carefully check operating times for cable cars, lifts and attractions; many are on maintenance in November or closed for the winter season. November is the least best time to travel to Switzerland because of the before-mentioned reasons and because the weather is usually very unpleasant (low fog in towns, rain, short & cold days).
Best regards,
Roland