The first year Norwegian worked with Nickelodeon, they had selected ships and itineraries that features Nick characters and events on every sailing. In addition, they operated one or two departures with live talent from the TV channel, effectively a much "beefed-up" Nick cruise.

Now what seems to be happening is an enhanced program of events on every departure, but they don't seem to be running the special "super" cruises.

More info here:
http://www2.ncl.com/freestyle-cruise/nickelodeon

As to the OP'er, hard to say without knowing a little more about what you're after. I worked for Norwegian for several years and know the Gem, Epic and Jewel very well. I like all of them - having sailed Norwegian Epic last, I have a soft spot for that one. It's really very good indeed for kids, especially the water park, and there are some fantastic restaurants onboard.

The itineraries are pretty-much year round, but only on the regular roundtrip itineraries, not the repositioning cruises.

Any more info, just shout.

Mark