Leaving Oslo and Headed to Bergen - how to go, where to stay, what to do!

I can't give you any advice related to traveling with children, but I would suggest you look at the NiN website. You don't have to book there, but it will at least give you some ideas on the options available. And it's certainly easy to book there, as it can all be done as a package. You pick and choose the components (trains, ferries, buses, even hotels if you want) and pay once. Then you pick up your tickets a day or two ahead of time at the Oslo train station.

It doesn't need to be confusing. And here's the good part: whatever you see, wherever you go, it's some of the most beautiful scenery you can imagine.

I see on their website that they have a new tour this year: "Sognefjord in a nutshell & UNESCO Fjord Bus tour." And there are the traditional NiN and SiN and HiN.

After you look at the options, post again with any additional questions you have.

Enjoy your trip!

The Norway tourist website may give you some ideas:

https://www.visitnorway.com/plan-your-trip/getting-around/by-boat/fjord-tours/norway-in-a-nutshell/

https://www.visitnorway.com/places-to-go/fjord-norway/the-sognefjord-area/flam/

https://www.visitnorway.com/places-to-go/fjord-norway/voss/

Bring your hiking boots and study the Norwegian Mountain Code:
https://www.visitnorway.com/plan-your-trip/safety-first/in-the-mountains/

Plan your trip and inform others about the route you have selected.

Adapt the planned routes according to ability and conditions.

Pay attention to the weather and the avalanche warnings.

Be prepared for bad weather and frost, even on short trips.

Bring the necessary equipment so you can help yourself and others.

Choose safe routes. Recognize avalanche terrain and unsafe ice.

Use a map and a compass. Always know where you are.

Don’t be ashamed to turn around.

Conserve your energy and seek shelter if necessary.