We have been on several cruises, but I've never been tempted by the Norwegian fjords because I assume that (a) they all look roughly the same (beautiful, but all similarly beautiful), (b) the towns where you land will mostly be small and not have much to see, and (c) you have to cross the North Sea, and neither of us are fans of waves.
In August of last year we went on a Baltic cruise, starting from Copenhagen, and that was fabulous. We visited seven countries (it would have been eight according to the original itinerary, but St Petersburg is a bit out of favour at the moment), and there was a huge variety of ports and scenery. Plus the sea was very flat.
We went with Azamara which is "cheap luxury" - small and quite upmarket, but understated and a lot less expensive than Regent or SilverSea. But you can go with TUI Marella (which I have heard called "Butlin's on Sea" — I have no idea how [un]fair that is), MSC ("Eurobutlin's on Sea"), and Aida ("German Butlin's on Sea"), through the big American brands like Royal Caribbean and Disney, then P&O and Cunard if you want "naice" and British, all the way up to Four Seasons (not quite as expensive as your own yacht). Some have great entertainment and stuff organised for kids — Azamara's entertainment was minimal and there was zero laid on for kids, but that's fine by us.