I am starting to sound like a broken record with repeats, but I am going to suggest an alternate route from Mürren to Italy that is easier than the Bernina Express, and to my mind equally scenic. Plus there is a wonderful family-run hotel on the way, and an option for a boat ride as part of the journey.
The alternate route is to take the scenic Treno Gottardo from Luzern to Locarno. The route is shown as #5 on this Swiss rail map which highlights all the named scenic routes. (You can see from the map why is takes so long to get to the Bernina express from Mürren—-it is far to the east).
https://cdn-doc.myswissalps.com/docs/default-source/rail-network-maps/swisstravelpass.pdf?sfvrsn=7788603a_62
This means putting Luzern after the Berner Oberland, not before. So from the Zurich airport you go to Bern for an overnight instead of Luzern. We did this our last trip (this past September) and found Bern to be a charming and very friendly town. Then you trip to Mürren the next day will be quite short.
After Mürren, take the scenic Golden Pass route to Luzern. Spend 2 nights there. On the intervening day, you can go up to the Rodelbahn (summer luge) and rope course that is partway up Pilatus:
https://www.pilatus.ch/entdecken/rodelbahn-fraekigaudi
Then take the boat the short distance to Weggis, at the base of Rigi, and stay at family-run SeeHotel Gotthard on the lakefront:
https://www.gotthard-weggis.ch/en/
This was our favorite hotel on our recent trip. My son and his family had a family room in the covered, in the little turret you can see in some photos. We had a room with balcony facing the lake. They have a lakeside restaurant and a more casual pizza restaurant, and we enjoyed both. And guests here are free to use the outdoor pool at the hotel next door, accessed by a back gate from the dock.
There is a cablecar in Weggis that will take you up to Rigi Kaltbad on Rigi, with numerous opportunities for short or long hikes, a cogwheel train, and childrens’activities.
https://www.rigi.ch/en
From Weggis we continued by boat down the lake to Flüelen, where we met the Treno Gottardo for the rest of the journey to Italy. The lake cruise was another highlight of our trip.
Our original plan was then to stay in Locarno 2-3 nights at Hotel Geranio au Lac, right on the lake a short walk from the train station.
https://www.hotelgeranio.ch/en/
This did not work out for us due to weekend track closures between Milan and Verona, our next planned destination, but I will suggest it to you as a possibility.
I will be back with suggestions for good playgrounds around Mürren and Wengen, but now we have to go pick up our granddaughter!