I don't know this for certain, so would certainly check directly. My cruise wasn't a themed one but they had events, but it really wasn't a Nickelodeon cruise, not marketed as one, and as such you wouldn't be wise to market it to your kids as one either as if they are expecting it to be wall to wall its not... just some key events throughout the cruise.

If you go for the Epic, and confirm that the Nick guys are on there full time, I would just say 'we're going on a cruise' then surprise them with the Nick events, otherwise they might be disappointed.

I'm guessing that there were about 4-5 events through a 7 day cruise (such as slimetime, poolside games, etc) but that is purely a guess from memory and there might be a disgruntled Nick employee coming along to say they worked a lot harder and did a lot more...

Epic is a big ship, but very kid friendly in my mind.