8 nights in Switzerland

Two nights in the Lauterbrunnen Valley is rushing it, especially since you’re in the mountains when the weather can suddenly change what you planned for the day. I suggest five nights so you can absorb it all. There’s a lot to do in this valley and I suggest slowing down and seeing it. Things to do and experience in the Berner Oberland can be found here: https://www.ricksteves.com/europe/switzerland/lauterbrunnen-valley.
The same with Zermatt, it’s best to add an additional night and I don’t know if three nights is enough because there’s a lot to do here too: https://www.ricksteves.com/europe/switzerland/zermatt.
Bern is convenient if visiting the Lauterbrunnen Valley. Are your kids good swimmers? Swimming in the river Aare is a popular sport during the summer months: https://en.wikivoyage.org/wiki/Berne.

You do not need to spend 2 nights in Zurich to catch an early morning flight. I suggest your time is better spent elsewhere, so I will suggest a 7-night itinerary.

You say you want to spend time riding scenic trains, but the standard Luzern +. Berner Oberland trip (Wengen, etc.) includes only a small section of the scenic Golden Pass route. The rest of the travel, while it may be scenic, may include tunnels or flat areas that are not what most people think of they hear of scenic train journeys in Switzerland. While 2-3 nights in Luzern and 4-5 in Wengen (or other village) makes for a very nice trip, kind of the standard recommendation here, you could include some very scenic rides by moving around at bit, including Zermatt as well as other beautiful Alpine scenery.

Take a look at this map of Swiss rail routes, with the named scenic routes highlighted:

https://www.mystsnet.com/en/download/swiss-travel-system-map-en-2023/?wpdmdl=14294&masterkey=63a023c8da36c

Note also the upper panoramic map, which shows the topography of lakes and mountains.

Your itinerary would look like this:

Day 1 is Paris to Bern to Luzern, spend 2 nights. Use the intervening day to do the Rigi round trip by boat, cogwheel train, cablecar, and another boat from Luzern to the tip of Rigi and back. Or a similar tip up Mt. Pilatus (b;ut the Rigi one is fully covered with a Swiss Travel Pass).

On day 3 take the scenic Treno Gottardo from Luzern over the Gotthard Pass to Locarno on Lago Maggiore in the Italian-speaking canton of Ticino. You will arrive in time to have the whole afternoon to explore the lake, or even ride a Postbus into the Valle Verzasca to the famed Ponti dei Salti. Or borrow bikes from the hotel and ride the lakefront path.

https://www.ascona-locarno.com/en/explore/valle-verzasca

Spend one night in a lakefront hotel just 2 blocks from the train station, such as this one:

https://www.hotelgeranio.ch/en/

Day 4 you ride the scenic Centovalli scenic train to Domodossola and pick up the train heading north to Brig, changing there for the train to Zermatt, which uses the scenic Glacier Express route.

https://www.myswitzerland.com/en-us/experiences/vigezzina-centovalli-railway/

Spend the next 2 nights in Zermatt. Then head to Wengen for your last 2 nights before trailing to Zurich the evening before your flight.

I will want you that riding trains in Switzerland is expensive, so you will need train passes to do this. But it does accomplish 2 of your goals, the scenic train rides and a visit to Zermatt, that are not included in the standard Luzern plus Berner Oberland itinerary.

Here is an alternate plan that includes Zermatt.
Go all the way from Paris to Zermatt in one shot. Its 8 hours on trains. Leave early and arrive early afternoon. Stay 2 nights
Zermatt to Grindelwald (or Wengen), If you like, take the scenic route via Kandersteg, but it will take at least 1 1/2 hours longer than the fast way. Stay 3 night nights.
Train to Luzern, stay 2 nights. the train from Interlaken Ost to Luzern is very scenic.
Late train to Zurich. Stay 1 night, then to the airport.

Thank you all for the advice!