NCL has the contract with Nickelodeon so you will only find the Nick characters on NCL ships. Not year round since the sailings are usually concentrated around dates when kids are out of school.

Disney is good if you like Disney. Lots of opportunities to take pictures with the characters. And the photographers (at least all the ones I've asked nicely) will even use your own camera, unlike the other cruise lines.

Disney cabins also have the split bathroom: toilet in room, tub/shower in the other.