Glad I could help.
You are wise to consider jet lag/travel exhaustion, epecially with children.
If you use the timetable, you'll find plenty of trains to Zermatt, with various routings. Since you will have to go through passport control, I'd count on getting a train more than one hour after you get off the plane. It usually a bit faster, but an hour should be safe. The train station is 3 minutes walk from the exit of the arrivals hall, where you pick up your luggae and exit through customs. It is on a lower level; the ground floor has an extensive food court. All other things being equal, you might try to find the route with the fewest changes of train.
You'll probably want a rail pass, and if you are able to print one at home ( usually possible) it will save you time at the airport:
I'm not an expert on the passes, but if you have questions, Arno or Annika will jump in. There is some kind of reduced fare or free pass for children of certain ages.
I've attached a picture of my 8 year old granddaughter enjoying the walk from Männlichen that I mentioned above. (Also some at Männlichen). Even though it is spectacular, it is quite easy; so easy that it is nick-named "The Grannies' Walk." And, even if you do not take the walk, it is a nice trip to go up and have a meal or a coffee, for example, from Wengen, then go down again.
You'll also see plenty of nice scenery on your chosen routes.
The Golden Pass Route from Montreux to Interlaken (and on to Luzern) is quite nice:
To see the schedules in the timetable for the trains along the route, make sure to enter "Zweisimmen" in the "via" box. Otherwise,the timetable will default to the faster and less scenic routes, such as via Lausane and Bern.
Between Interlaken and Luzern, the Luzern-Interlaken Express follows the Golden Pass Route. It goes via Meiringen. However, I think that that is also the fastest route, so it will probably be the default in the timetable.