Hello Coigoufo34
Phew, I usually I keep my advice limited to public transport related topics, however, with your question, I feel obliged to look into your overall trip planning, if you allow me to do so.
Two days Switzerland whilst you wish to travel from Tirano to Geneva? This is, sorry my French, insane, unless the main purpose of your Switzerland trip is to experience the Swiss trains in full - my brother would definitely fall into that category of travellers.
Although I feel honoured as a train staff that you intend to travel all by train, your travel plan means, you will spend both days in trains solely; to put it bluntly, I am unsure if your kids will still love you after this trip 🙂 . It means further, you will have seen a lot of different landscapes our country has to offer, yet you will not have experienced it at all, including the mountsins - for what our country is totally famous. Plus, I fear that you forgot the needs of your kids; if they love to travel by train or love trains in general, the trip is fine; otherwise, you should remake your plans.
Have you verified the timings and time needed for this trip using the official trip planner for Swiss public transport, www.sbb.ch/en?
I would have suggested, to travel to Lucerne from Italy directly, where a lot of family activities are possible, such as a cruise on Lake Lucerne on the old steam paddle boats - I remember that this was astonishing for me as a child, and still is nowadays as an adult; you can experience the steam engine making the boat move forward. Or, tobogganing near Fräkmüntegg, after having visited Mt. Pilatus using the steepest cog wheel train in the world. Or, simply travel directly to Interlaken from Italy, which also offers a lot of family activities; the near lakes of Thun and Brienz, both offer a great lake experience as well. There, you can also experience the majestic mountains being named Eiger, Mönch, and Jungfrau. From my own experience, the Youth hostel in Interlaken is a great and budget friendly place for a family overnight stay.
See:
https://media.jungfrau.ch/image/upload/ar_16:8,c_crop,f_auto,fl_lossy,q_auto/c_scale,w_2309/v1479841709/fileadmin/Winteregg_Muerren/Winteregg-Muerren-Restaurant-Sommer-Panorama-Eiger-Moench-Jungfrau.jpg)
https://www.bls-schiff.ch/en/lake-cruise/good-know/about-lake-brienz
To conclude, your trip is fine as such yet would definitely need more time in order to feel the vibes of our country; hence, whenever you can, assign at least 3 to 4 days in Switzerland to experience the beauty of our country.
If you insist and want to stick with your original plans, yes, the Saver Day Pass is the best option for only two days. And, a children day pass for the youngest. The Junior Card for tourists is not available online, you would have to find a railway office in Switzerland to purchase it.
If you need help with planning your trip or want to have a second opinion on your travel plans, I can warmly suggest TripAdvisor’s Switzerland travel forum run by local volunteers. Many of them have or had kids and can give you a more detailed advice on travel in Switzerland with children: https://www.tripadvisor.com/ShowForum-g188045-i336-Switzerland.html
Please let us know if you have more queries.
Best wishes,
Roland