Hello again ahm
I am now assuming that your party consists of two parents, and two children aged 10 and 12. The children will need tickets for the Italian part of the journey, Milan [??Malpensa Airport??] to Tirano.
Invest in Swiss Travel Passes for the two grown ups. Get the FREE Swiss Family Card (https://www.myswissalps.com/travel/tickets/discount/) for the children. This in turn will mean that the children will travel FREE in Switzerland. BUT, if you are travelling on one of the FEW trains in Switzerland which require seat reservations, you will have to pay for FOUR seat reservations.
Whether you can drop the one night in Milan really depends on the time you expect to reach Milan. Which airport are you flying into (Milan has three!) and when do you touch down?
If you are able to drop Milan, the grown-ups may need 8-day STPs commencing 3rd April; these are 419 CHF per adult, second class. (6-day STPs are 379 CHF)
Bernina Express v Ordinary Train.
The scenery is the same whichever type of train you travel on! The Bernina Express (https://www.myswissalps.com/activity/bernina-express/) runs twice a day. The ordinary trains Tirano to St Moritz are approximately hourly. Journey time is about the same. The Bernina Express has luxury ‘Panorama’ carriages which are ARGUABLY better to travel in than the ordinary carriages on ordinary trains. Seat Reservations ARE compulsory for the Bernina Express, and cost 36 CHF per person. So that is 144 CHF for your group of FOUR to travel in ?’better’ carriages.
Glacier Express v IR38 Train
The scenery is the same whichever type of train you travel on! The Glacier Express (https://www.myswissalps.com/activity/glacier-express/) runs twice a day. The IR38 trains are exactly hourly. Journey time St Moritz to Chur is about the same. The Glacier Express has luxury ‘Panorama’ carriages (some of which are 30-years old) which are ARGUABLY better to travel in than the modern five-year old carriages on the IR38 trains. Seat Reservations ARE compulsory for the Glacier Express, and cost 49 CHF per person. So that is 196 CHF for your group of FOUR to travel in ?’better’ carriages.
The IR38 trains get to St Moritz at 53 minutes past each hour, and leaves to go back to Chur at 07 minutes past the hour. Be on the platform at St Moritz by 52 past the hour. As soon as you can, grab a group of (unreserved) 4-seats on the LEFT-hand side of the train in the direction of Chur. That is the better side for the scenery.
Lauterbrunnen (https://www.myswissalps.com/town/lauterbrunnen/)
That village is best visited on your way to or from the Jungfraujoch.
Children’s Activities.
You are travelling in early April. There will still be a lot of snow at higher levels. But playgrounds etc are not my field of expertise.
Rhine Falls (https://www.myswissalps.com/activity/rhine-falls/)
These are just 48 km (29 miles) north of Zurich. If your flight from Zurich Airport is later in the day, check in your bags at the Airport in the early morning and then go to Schloss Laufen am Rheinfall. Take any train to Winterthur, and change there for Schloss Laufen. Aim to be back at the airport at lest two hours before take off. Trains from Schloss Laufen run twice an hour, and the journey (including the change at Winterthur) takes about 45 minutes.
Best wishes for your detailed planning.
Roger