Paris to various Swiss destinations - are these the best train routes?

2nd class is always good. The only reason IMO to go to 1st is if it is only few EUR/CHF more.

Take the most direct route, likely Paris to Basel to Interlaken to Lauterbrunnen to Wengen.

Look at the Half Fare Card plus the free Family Card so the kids travel free with you. If total undiscounted tickets per person is 240 CHF or more, it pays off.

Sara, The best route is usually the quickest. If you look up A to B in one of the train company websites (for example the Swiss Railways one: https://www.sbb.ch/en ) that will give you the quickest option. There are two routes from Paris to Switzerland: via Basel and via Geneva/Lausanne. For Wengen, the quickest option is via Basel.
The current Paris - Switzerland timetable is here: https://www.tgv-lyria.com/sites/default/files/pdf/BROCHURES_CO/lyria-timetable-10.12.17_06.07.18.pdf As you can see, there is one train every 2 hours. You will need to change trains in Basel, Interlaken Ost and Lauterbrunnen.

"Advance purchase for Swiss travel needed?" - no, never needed. There are some discount tickets for lesser used trains, but not many. Reservations are never needed (except for the 4 named tourist trains).

First class - don't bother. For the TGV Paris to Basel you can sometimes get special offers where 1st class costs €10-15 extra, otherwise don't bother. For more info on the trains used on this route, including photos of 1st and 2nd class, and a video, see here: https://www.seat61.com/lyria.htm
Equally don't bother with 1st class in Switzerland, 2nd is totally comfortable.

Ticketing in Switzerland is complicated. The options are:
1) Just pay full fare. Kids (6-16) ½ fare.
2) Get a Half fare card (CHF 120 per adult), and pay half fares. See here: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-halffare-card.html Note the comment about kids 6-16 being free.
3) Get a Swiss Pass and pay nothing extra: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-travel-pass.html Note kids also travel free,.
4) Berner Oberland Pass: https://www.regionalpass-berneroberland.ch/en/ This gives better coverage of the mountain railways above Wengen, at the cost of not covering Basel - Bern or Bern - Zürich airport. Kids pass CHF 30.

Without doing the arithmetic (which is boring), I guess option (2) - Half Fare Card, is your bets option.

If the routing through Lausanne is markedly cheaper than through Basel, don't count it out -- just as easy, and you will have to transfer to get to Interlaken anyway.

When thinking about the Half Fare Card, don't forget the substantial cost of mountain cable cars (Jungfrau or Schilthorn) which are both half-price using the HFC.

Thank-you to all for this helpful information!!