Hello Arhanning
Welcome to MySwissAlps
You are not too late to book trains for your proposed journeys.
Swiss Travel Pass and Swiss Family Card
It will be easier if I deal with these passes first before I look at routings from/to Italy.
In the space of four days, you will be doing quite a lot of travelling within Switzerland. That fact, and the age composition of your party, suggest to me the following.
For you yourself invest in a four day Adult Swiss Travel Pass.
For the 16 year old invest in a Swiss Travel Pass ‘YOUTH’.
For the 14 and 11 year olds get a Swiss Family Card. Provided you have got an STP, the SFC is FREE.
https://www.myswissalps.com/travel-ticket/swiss-travel-pass/
https://www.myswissalps.com/details/buy-swiss-travel-pass/
The Swiss Travel Passes for the older members of the group will give you free travel on all Swiss public transport except lines which simply go up mountains. On those lines the STP USUALLY gives a 50% discount, though to the expensive Jungfraujoch the discount is only 25%.
The Swiss Family Card for the two younger members of your group gives them free travel on all Swiss public transport INCLUDING the high mountain lines, provided you (the parent) are with them.
‘On the SBB website it says that trains are not available for some of my routes.’
What ‘routes’ were you envisaging?
Get out a good map showing northern Italy and Switzerland.
There are two main routes between Italy and Switzerland. The Gotthard route via CHIASSO and Lugano. The Simplon route via DOMODOSSOLA and Brig.
The Gotthard route is longer, but the trains are much more frequent. The Swiss Travel Pass and Swiss Family Card are valid from Chiasso, which is only 51 kilometres (31 miles) north of Milan.
The Simplon route is a bit shorter, but the trains are less frequent and sometimes disrupted by engineering work. Moreover the Swiss Travel Pass and the Swiss Family Card are valid only from Domodossola, which is 125 kms (77 miles) from Milan.
https://www.myswissalps.com/travel-guide/italy-switzerland/
October 10th Florence (Firenze in Italian) to Interlaken Ost.
I have just gone to the journey planner on http://www.sbb.ch and keyed in Firenze S M N as my departure point and Interlaken Ost as my destination. I have keyed in OCTOBER 10TH as my day of travel. I selected 07 30 as my earliest start time.
It gives me a very attractive routing leaving Firenze S M N at 07 55 on a Frecciarossa high speed train, arriving at Milano Centrale at 09 50. From there you catch a Eurocity Train departing at 10 10 bound for Basel. You stay on this train as far as Luzern (known in English AND French as Lucerne). You reach Luzern at 13 41.
Catch the 14 06 Luzern-Interlaken Express; this reaches Interlaken Ost at 15 54.
Tickets
You will need Italian tickets for all four of you from Firenze to CHIASSO. Your STPs and SFC will take you the rest of the way. (You stay on the EC train when it stops at Chiasso.)
Seat Reservations
You will need four seat reservations on the Frecciarossa, and four seat reservations on the Eurocity from Milano to Luzern.
If you want to be sure the four of you are sitting together, reserve seats on the Luzern-Interlaken Express. (Friday afternoons are busy.)
https://www.myswissalps.com/activity/goldenpass/
https://www.myswissalps.com/travel-ticket/seat-reservation/
October 12th Interlaken to Zermatt.
Consulting the journey planner on the SBB website it is showing exactly what I expected. Basically you leave Interlaken Ost at 29 or 59 minutes past the hour, and changing at Spiez and Visp it takes about two hours twenty minutes to get to Zermatt.
https://www.myswissalps.com/travel/public-transport/timetable/
Your STPs and SFC will take you the whole way. No need to reserve seats.
October 13th Zermatt to Milano
The journey can be done easily if you travel in the afternoon. The Simplon Railway from Domodossola to Milan is closed in the morning for engineering work; but it will be open in the afternoon.
I entered into the journey planner on the SBB website a journey from Zermatt to Milan starting not earlier than 12 00 on 13th October.
There are several possibilities shown. The best is as follows.
Catch the 13 03 R40 train from Zermatt to Täsch, arrive 13 14.
Catch 13 21 bus from Zermatt to VISP station, arrive 14 18.
Catch the 14 33 Eurocity (via Brig and Domodossola) train to Milan Centrale, arrive there 16 55.
Tickets
You will need Italian tickets for all four of you Domodossola to Milano.
Seat Reservation.
Reservations are not possible on the R40 or the bus. You will need seat reservations on the Visp to Milan Eurocity.
Best wishes for your detailed planning.
Roger