Advice required with Swiss pass or direct purchase from Paris

I assume you mean the "Swiss Family Card". This is free, details here: http://www.swisstravelsystem.com/en/passes/swiss-family-card.html

If you are taking the train from Paris to Luzern (local spelling) you will change trains at Basel. Basel is the border station between French and Swiss railways, Swiss tickets are valid from there. The simplest option is to buy your tickets from Paris to Basel online. When you change at Basel get your Family Card and any other tickets you need then, and then catch the next train to Luzern. There are trains every half hour from Basel to Luzern. There is no price advantage to buying Swiss rail tickets in advance, neither are reservations necessary, tickets are valid on any train.

I assume you have priced up a Swiss travel pass (details here: http://www.myswitzerland.com/en-ch/swiss-travel-pass.html ) and a Swiss half price card (details here: http://www.myswitzerland.com/en-ch/transport-travel/tickets-for-guests-from-abroad/swiss-half-fare-card.html ), and have decided in your case it is cheaper to pay full fare.

P.S. It is Jungfraujoch, not "Jungfrach deck"..
If you are planning to travel by train from Luzern to Mürren, Gimmelwald and Jungfrau, you may well find buying a Swiss half price card pays off.
But, all these three places are near each other, but a long way from Luzern. Look at the map here: https://www.swisspasses.com/railpass/overviewmap_en.pdf
The route from Luzern to these places is scenic (over the Brünig pass), but doing it every day would get repetitive. If these are your planned goals, it may be better to stay in that area, rather than commute.

If you stay in Luzern, I recommend going up Titlis as a day trip. Details here: http://www.titlis.ch/en

Thank you Chris. I am thinking of the following plan. I am yet to make reservations.
Since we are 2 Adults + 2 children, even if I invest in Family pass for 4 days, I will have flexibility of local transportation and trains, also will get some discount on the mountain railways to Jungfraujoch, and on boat rides at Luzerne. As you suggested, I will workout the direct ticket costs and then finalize on the swiss pass.

26th May: Paris to Luzerne via Basel. Buy Family pass in Basel. Cover local places in Luzerne as much possible

27th May: Trip to Mount Titlis

28th May: Depart to Interlaken. Cover Jungfraujoch Observation deck. Stay in Interlaken

29th May: Visit Murren or/and Gimmenwald. If possible stay in Murren overnight otherwise Interlaken

30th May: Interlaken to Montreaux using the Golden train or normal trains(not sure of availability, may be they go through Bern, if so, I will not be able to cover the scenic route.)

30th May: Montreaux to Geneva Airport. Depart from Geneva to Norway.

please suggest/correct.
thanks a ton,
Raj

Yes, look at either two 4-day Swiss Travel Passes to begin May 27 to May 30, or two 30-day Half Fare cards. the Family Card is free with both and the kids ride free, provided they are under 16.

The way to decide is to put a spreadsheet together and see what saves the most money. One point to keep in mind is the Jungfraujoch. The Travel Pass covers the trip to Wengen, but only a 25% discount of the journey from there to the Jungfraujoch. The Half Fare card covers a 50% discount for the whole trip.
Use www.sbb.ch/en for prices and also prices for Travel Passes and Half Fare cards.

So if you opt for the Travel Pass buy tickets from Paris on-line ahead of time all the way to Luzern. The sooner, the better, as the price only goes up. Try www.capitainetrain.com. You can buy the Passes and Family Card in Luzern. The Travel Pass comes in 3,4,8,and 15 day versions.

If you opt for the Half Fare cards, buy the ticket as far as Basel, then buy your Half Fare cards and Family Card in Basel, then continue on to Luzern.

Should be no problem traveling Interlaken to Montreux on the Golden Pass route (as opposed to the named trains) as there are trains at least every hour in Switzerland.