Murren to Zurich, and how much time in Zurich?

You can look at the schedule and the routings on the SBB website. It appears that there are two departures per hour. You'll be taking the cableway down to Stechelberg, walking a short distance to the bus stop, taking the bus to Lauterbrunnen, then taking 3 or possibly 4 trains on to Zurich. The entire trip takes about 3 hours. The trains are coordinated so you won't be doing much standing around. The most important thing is to have everyone ready to hop off those trains when they arrive at the transfer points (Interlaken Ost and Bern, plus possibly also Spiez). You shouldn't have a problem getting onto the next trains, but if something does go wrong, there will be another train along in 30 minutes, so no big deal.

I always think more time in Europe is better. Whether Zurich is the best place to spend two extra days, I am not sure. I haven't been there recently and in the past, I preferred Bern.

I would skip Zürich. Perhaps add the two nights to Mürren, or one to Mürren and one to Bern (say hello to the bears).

If looking up times on the aforementioned SBB website, if you are going direct to the airport you want the "Zürich Flughafen" station, under the terminal. The one in the city centre is "Zürich HB". There are 2 trains per hour from Bern to Zürich Flughafen, and it takes about 1h20.

Just spend the last night in Zurich (not at Zurich Airport) before you fly home.

Don't spend extra time in Zurich when you could be in Murren. There is no comparison!!! The train connections are frequent and efficient.

There are two ways to get from Mürren to Lauterbrunnen, where you will then make your way to either Zürich or Zürich airport. The way given by acraven is from the Mürren Schilthornbahn station.

At the "other side of town" (it's only a 10 minute, almost level walk) is the Mürren BLM station. From this, you take the train to Grütschalp (15 minutes), then the cable car down to Lauterbrunnen (5 minutes). Then you cross the street to the Lauterbrunnen train station.

Both methods work fine; choose whichever one is easiest for where your hotel is located. And both methods may sound a bit complicated now, but once you're actually there, you'll see how easy they are. All the connections are coordinated and unreserved, and all run several times an hour, so you just get on the next one (bus, cable car, train, etc).

What time is your flight likely to be?

You and your SO, stay in Murren and leave for Zurich in the morning.

You, SO, AND three kids? Stay in Zurich at Apart-hotel operate by Hilton in Opfikon, 5 mins from ZRH. Seriously.

The OP doesn't indicate that he is trying to get to Zurich for a same-day flight.

Thanks everyone. Very much appreciated

It's very easy to get from Mürren to Zürich, as the Swiss have one of the best and most punctual transportation systems in the world. Travel time will be about 4 hours with a minimum of 4 changes (that sounds more complicated than it is). There will be numerous trains every day, so you can choose your departure time.

Are you aware of the route you'll need to take to reach Mürren? Once you reach Lauterbrunnen, there are two route options and you'll need to know which one you'll be taking when you buy your rail tickets.

If you have the time, I'd suggest spending 2 nights / 1 full day in Zürich, so that you can have a brief look at the sights. It's a beautiful city but like any in Switzerland it's expensive. You could consider taking one of the small bus tours of the city, or perhaps a cruise on the lake. This website may provide some ideas on things that might interest your group.....

https://www.zuerich.com/en

You should have a better idea on how many days / nights to allocate to Zürich after deciding on your sightseeing activities.