Seeing Norway on Ocean Cruise versus NIN

One better option if considering a ship is Norway’s Hurtigruten ships. You see a lot more than what you mentioned for a cruise ship. NIN would be great, combine with a Hurtigruten out of Bergen

This is a very personal decision; I love both forms of travel.

On a cruise, except for the time you are in port, it's all about the cruise, a floating city: familiar foods on board, maybe even the same table in the dining room every meal, waited on by very well-trained and attentive staff, homogenized cabins, English TV and movies in the cabin, live shows mostly in English, music venues, kids clubs, babysitters, spa, bars, even a casino. Count the number of hours your chosen ship will be in port. Remember a ship can miss a port due to weather; my trans-Atlantic missed Bermuda due to a storm. Now assess out how much you are paying for the time actually spent in the countries.

The land travel is more challenging mentally and physically, but you see more and are immersed in the culture and will learn more. If you are having difficulty putting the land travel together, perhaps the children would like to read and make suggestions.

Seeing as how you are taking a trans-Atlantic after this, you might want to think about the amount of cruising you want to do. You will have 7 straight days at sea already.
BTW for the trans-Atlantic, I recommend a non-fiction reading, Simon Winchester's "Atlantic".

I've only done the traditional NiN train from Oslo to Myrdal to Flam then boat to to Gunvangen then bus to Voss and train to Bergen. We did an overnight stop at Voss but there are multple options starting with no overnight stays to multiple stops. The NiN give you a cross section of Norway as you travel along river valleys, across a high palteau (Hardangervidda) and narrow fjords. A boat cruise would limit you to a water level view of Norway unless you can do an extra (eg. Flam train to Myrdal).

I haven't exprienced the Geirangerfjord version of the Norway in a Nutshell but the 4 plus day package seems to be the best of both:
https://www.norwaynutshell.com/geirangerfjord-in-a-nutshell/bergen/

Geirangerfjord and Norway in a nutshell® tour from Bergen

You start your tour in Bergen on the popular Norway in a nutshell®
tour, which takes you through some of Norway's most beautiful fjord
scenery. You will experience the scenic Bergen Railway, the
Aurlandsfjord and the narrow Nærøyfjord on a fjord cruise and the
breathtaking Flåm Railway. From Myrdal you take the Bergen Railway
across the country to the capital Oslo, where you have one (or more)
overnight stays, depending on your wishes.

From Oslo you travel on the Dovrebanen Railway through impressive
valley and mountain regions to Dombås. The trip continues from Dombås
on the Raumabanen Railway, one of Norway's most beautiful and wild
stretches of railway. The journey on the Raumabanen will take you over
the Kylling bridge and past the vast and impressive Trollveggen cliff.
A bus will be waiting for you in Åndalsnes to take you to the Art
Nouveau city of Ålesund where you can stay one or several nights,
depending on your wishes.

The trip continues with a fjordcruise on the magical Geirangerfjord
with Hurtigruten.