I did the journey from Tirano to Chur and back in February 2023. I used normal service trains only, changing at St Moritz.
I bought the tickets at the ticket office at Tirano, 1 hour before boarding the train (get there early to avoid queues). My plan was to travel to Chur on day 1, stay overnight at Chur and return to Tirano on day 2. The ticket clerk suggested a buy a 2-day ticket, which was valid on the whole RhB network, which was about 10 CHF cheaper than buying 2 singles.
On the Tirano to St Moritz journey, the train was quiet in the first half (I left at about noon). I could easily swap sides on the train to get the best view. It was an Allegra and I opened the windows on both sides of the train to get photos. But once at the peak, a lot of people got on and all seats were occuped. An elderly couple shut all the windows on the train when boarding. Then I re-opened the window adjacent to me. After a while, the elderly couple came to me and complained about the window and saying it was cold (they seemed to be regular travellers/locals). I reluctantly closed the window for the remainder of the trip. The heating of the train was very warm, everyone was in T-shirts.
From St Moritz to Chur and back to St Moritz (I know this part of the trip is not relevant to the OP), the train has a photography carriage, which is specifically designed with electric opening windows for taking photos. I stayed in that carriage and was much happier. These trains also have a restaurant car.
On the return trip from St Moritz to Tirano, the people seemed to be all tourists and didn't mind when I opened the windows.
Anyway, to the OP, I think it depends on what you and your family prefer. If you want a peaceful ride without moving around, you could go for the panoramic cars (All services have panoramic cars***, even the non-Bernina Express trains. You can pay the supplement onboard). But if you plan to take a lot of photos, the regular service trains might be better with opening windows.
In the summer, they attach open-top wagons to the Tirano - St Moritz trains too.
If you do decide on the panoramic cars, the general recommendation is to choose a seat on the left side of the train when going North, and right side when going South. You won't be able to figure out the direction of the train if you choose your seats online. So the best idea might be to catch a non-Bernina Express train, board the panoramic car early to choose the best seats, and wait for the ticket inspector to come and pay the supplement.
Also, download these 2 brochures and load it onto your phone so you know what to look out for along the route. You should be able to find the brochures physically at Tirano and St Moritz too.
***EDIT, after reading some the earlier posts carefully, it seems this is only true for off-season trains.