My DD has been at
DCL, Carnival and MSC kids club, MSC wins for her hands down. I can only attest to the European cruises though....MSC counselors are VERY engaging, take the kids all through the ship(chaperoned of course) they do stage shows, have contests, face painting, and have dedicated seats in the front of the theater for the shows at night. Teens have their own club but my DD is 7 so I can only speak for that age range.
My DD is afraid of the larger than life characters, but likes the princesses. We still enjoy the Disney theme though, but don't have to cruise for that. We equally enjoy our time at the WDW resorts, and with our resident discount, we get good rates and it's a 2 hour drive for us.
Just want to add MSC is a European line, so you will have kids speaking many languages. The counselors speak English though as well as other languages. I thought it was a good cultural experience for my DD. She understands Italian but won't speak it much, yet she made great friends on board..so kids have a way of communicating, even if it's nonverbal. I loved that towards the end of the cruise my kid was calling me "Madame" lol