Switzerland and Venice Trip with kids ( 6 yrs and 10 yrs ) ; 8 days

Standard suggestion, don't stay in Interlaken, but in one of the places further into the mountains: Mürren, Lauterbrunnen, Wengen or Grindelwald.
For a map of the area, to show where these places are see here (red = rail lines, black = cable cars): https://www.regionalpass-berneroberland.ch/assets/karte-und-partner/Regionalpass-Panoramakarte-2017.pdf
For information and accommodation see here: https://jungfrauregion.swiss/en/summer/

You have 2 nights Zermatt, 2 nights Jungfrau region. That is borderline too few. You are dependent on the weather, with only two nights there is a risk you have two cloudy days and you cannot go up mountains.

Be careful with spelling place names correctly. If you spell them wrong when you are looking for transport or accommodation on the internet, you will not get the results you want: Spiez, Brig, and in the Italian rail planner ( http://www.trenitalia.com/tcom-en ) you need to use the Italian place names: Venezia.

Add actual dates to your list, for instance, is Day 7, Friday, May 4?

I was thinking that if you go from Montreux to Interlaken (or thereabouts) via Zweisimmen, you would be spending less (assuming you get Half Fare Cards) and getting more scenery. However, it appears that there is track repair work going on that would would require a replacement bus between Montbovon and Chateau d'Oex. So a bus meets the train at Montbovon, every body gets off and boards the bus(es) and they take you to Chateau d'Oex and everybody gets off and boards the waiting train to continue. Its an option.

From Interlaken take the train to Zermatt, then on to Venice. You should be able to get advance purchase nonexchangeble tickets to Venice at a discount now.

Your current plan is perfectly acceptable. As far as railcards, you and your wife will probably save the most by buying 30-day Half Fare Cards for 120 CHF each, and you can get a free Family Card and your kids travel with you free at all times. The other option is to get two 8-day Swiss Travel Passes for 398 CHF each plus the free family card. For various reasons, I do not think it is as good a value as Half Fare Cards, although they can be convenient. But for the mountain journeys you are thinking about, they still only give a 50% discount, and only 25% for the Jungfraujoch. The Half Fare Card gives 50% for everything, including local buses, trams, etc.

From your other posts, sounds like you will be at a conference in Montreux and your wife and kids will be joining you there from Geneva airport. So you could buy your Half Fare Card when you arrive in Switzerland, and your wife can buy hers and get the free Family Card when she and the kids arrive.

Booking tickets to Venice, use Venezia S. Lucia as your destination station. If you intend to get these ahead for a discount, you can enter such information as whether you have a Pass or a Half Fare Card so those discounts can be applied. You might even want to email SBB to help as getting the correct discount for the kids escapes me. The 6 yo should travel free for the Italian portion of the trip and the 10 yo at half price. Discounts would only apply to the portion in Switzerland.