I will suggest a couple of itineraries that start in Luzern and end in Zermatt, and include a scenic railway ride somewhere in between. Here is a map of the Swiss rail system, with the named scenic trains highlighted, so you can follow:
https://cdn-doc.myswissalps.com/docs/default-source/rail-network-maps/swisstravelpass.pdf?sfvrsn=7788603a_56
I will start by noting that there are direct trains from the Zurich airport terminal right to Luzern (2 per hour) that take just over an hour. Since you will be getting on a train in the airport to go to Zurich, you may as well stay on the train a bit longer and go to Luzern. It will save you a hotel change and other hassles. (depending on the type of train you take from the airport into the Zurich main station, it can be complicated just getting out of the station,with lots of escalators and walking.). And there are more nice hotels within easy walking distance of the train station in Luzern than there are in Zurich.
Options for routes:
1). If the Berner Oberland is a “must” then the logical route would be Luzern (2-3 nights) to Interlaken on the Golden Pass scenic route (#7 on the map), changing there for the local train to Lauterbrunnen. Spend 2-3 nights there or in one of the charming car-free villages above, either Wengen or Mürren. I will say that Mürren offers a great playground (Flower Park) at the top of the Allmendhubel funicular, and a “children’s Adventure Trail with slides and tunnels from there back down to Mürren.
https://swissfamilyfun.com/allmendhubel-flower-park-playground/
https://play-basel.weebly.com/flower-park-adventure-trail.html
My grandchildren did the Adventure trail when they had just turned 6 and loved it (so did their parents). I don’t know if there is anything similar in Wengen itself, but there is a nice playground at the top of the Männlichen cablecar.
https://www.maennlichen.ch/en/summer/experiences/alpine-herdsmans-playground.html
From your Berner Oberland base, head back to Interlaken Ost and continue on the Golden Pass scenic railway (#4 on the map) to Montreux on Lac Leman. The most scenic part of the Golden Pass route is often deemed the downhill ride from the pass to the lake, with the panoramic view of the lake. Spend a night in Montreux and enjoy the lakeshore, and maybe visit nearby Chateau du Chillon the next morning. It is called a castle, but it doesn’t resemble the type of fairytale castles a 4-year-old might expect.
Next day take the train from Montreux to Zermatt. This is a journey of 2.5 hours, with a single change at Visp. Spend remaining time in Zermatt, leaving the last night before your flight to spend in Zurich. It is a 3 hour 21 minute train journey from Zermatt back to Zurich, and I do not recommend taking that on the day of your flight.
2). As above, but after Wengen/Mürren, take the train to Spiez, changing there to the local train to Kandersteg.
This is not a named scenic route, but it is pretty nice. Spend 2 nights in Kandersteg, a nice alpine village with a lovely lake above, reached by gondola. You can stay in town or better yet, in the rustic mountain inn by the lake. The rooms do not have private bathrooms, but the shared shower and toilet rooms on the hall are modern and spotlessly clean.
https://www.oeschinensee.ch/en/
After Kandersteg, continue shout on the train to Brig, changing there for the train to Zermatt.
The other options skip the Berner Oberland and head south from Luzern to Locarno, another scenic route. I will have to finish that later.. .