At
DCL it doesnt matter how potty trained your kid is. If they're not 3 they can't go to oceaneers. If they turn 3 half way through the cruise, they can register half way through the cruise.
I see you mention RCCL too: At Royal if your kid is completely potty trained and no pull ups and can have a conversation then they will usually do a trail bump up to the aquanaut (3-5 room) at around 2years and 9 months based on manager approval.
For both companies the nursery is for up to and including 3 year olds. So they have activities and toys there for them. If they're not independent then the nursery is better as they get individual care and activities.
At disney 3-12s are all together. At Royal your kids could be together in the 3-5 age group. However kids are free to do what they like at Disney, when they like. They want to do craft for 3 hours then go for It, they want to play with a ball right now they Can, they want to play computer games, they're always available. At Royal they do everything together. Free play time. They'll all do the craft. They'll all play the ball game. If they don't want to join in, there is no other option.
I've worked the kids clubs at both companies so can see the pros and cons for both. Neither is better or worse than the other, they both actually quite different, just depends what would work for your kids.