Switzerland to Venice trip ideas?

I'd spend all 9 nights in Switzerland because there is lots to see and do there. Squeezing in Venice would rush everything and you would do neither Switzerland nor Venice justice. Save Venice for another trip. A Swiss itinerary like this might work:

3 nights in/near Lauterbrunnen
2 nights in/near Montreux
2 nights in Zermatt
2 nights in Luzern

Or skip Switzerland and do Italy. You can tackle Venice and Florence nicely with 9 nights.

First, I would suggest staying in Murren or Wengen as they are much more scenic than Interlaken. Second, a round trip to Venice is quite lengthy, 6-7 hours one way. I would suggest just staying in Switzerland and seeing Lucerne and Bern. There is plenty to see and do in both cities. Since your parents haven't been to Europe it wouldn't be beneficial to spend so much time on a train. You only have 9 nights.

Most definitely skip Venice. It would be almost 2 days of travel to get there and back to Zurich. We flew out of Zurich last year on a trip that included Venice, the CT, Varenna on Lake Como, Luzern and Zurich. The Swiss rail system is extensive, so you have the ability to visit more places in Switzerland. If you want a visit to Northern Italy, the train station in Como on Lake Como is about an hour's trip from Zurich. Lake Como is gorgeous, and ferries are available to take you sightseeing. I'm not sure when in September you are traveling, but it would be good to check when the ferries don't operate as the weather changes. We were there in May, so it wasn't a concern.

As a side note, when you make hotel reservations in Zurich, avoid Hotel Bristol which is close to the train station. Rick liked it; we did NOT!

Following, my husband wants to go from St. Moritz to Venice but we won't have much time there with travel time and we will have 2 kids in tow. Not sure it's worth it.

Thanks everybody for the helpful insights and opinions, all good things to keep in mind!

I know there is definitely plenty to explore around Switzerland, but if we do decide to try for Venice,
I'm also toying with the idea now if we should maybe from Bernese Oberland, train to Geneva, and catch a flight to Venice. Then stay Venice maybe 2nights, then fly back to Zurich the day before our flight.

Would that be doable? or still rushed?

Still rushed.

I agree with the others. I fully understand that for someone who's never been to Europe, it feels somehow incomplete to see only one country on a trip. But when you only have 9 days, and when you're in the mountains (so transit takes longer than on flat landscapes), you just don't have time for long detours. Plus, there's more to see in Switzerland - Zermatt, Luzern, Bern, Lausanne, Chateau de Chillon...

If you HAVE to see another country, look at places in France or Italy or Germany that are closer to Switzerland than Venice. Unfortunately, I'm not aware of any "iconic" sights that are close, except for the Last Supper in Milan. The problem is that Last Supper tickets are very hard to get; you can look into a guided tour around Milan that includes these.