@Yes, CJean, I got that. It was an example. I was thinking of an example that Roberto used this morning where he was talking about how Rome and Venice are well connected. My point was, Venice can and does have midday flights, and before I'd start planning an itinerary I would look at flying into/out of both airports, Zurich AND Venice. Since you did find it confusing, I'll change my 1st post.
I've flown into Zurich a few times and it does have its advantages. It is much easier and quicker to get to Luzern, city center, for one. So, at the start of a trip when the travelers are jet lagged, it is easy to hop on a train from the airport, then switch trains at Zurich HB and soon be in Luzern city center. But again, it would depend on whether OP can get a desirable flight out of Venice.
In Venice, many on the forum, including myself, like https://www.cortecampana.com
I stayed in Luzern with my two daughters. We stayed at Hotel Drei Konige. I appreciated the large family sized room, huge bathroom. We loved all the breakfast choices. Staff was friendly and helpful, hotel was quite clean. Some would call it "dated". But, the price was good. https://www.drei-koenige.ch
I found my Varenna accommodation on booking.com. It was fine, but I wouldn't stay there again. I would start looking for Varenna accommodation well before you think you need to.
I think the locations you've chosen are great for your aged kids. I think they will be wowed!