Welcome to cruise-chat! Well, the first thing to consider is the importance of having the two children be able to participate in the children's program onboard. If that is a high priority, although I am a huge fan of Royal Caribbean's youth program, their Adventure Ocean program is restricted to children 3 and older. Between the remaining two choices, either would probably suit you needs, but again there are some major differences. Your group of 12 might prefer the freestyle dining on NCL, where you can eat when and where you want (of course some of the options require a supplemental fee). Carnival has a more traditional dining program where set times and tables are assigned in the main dining room. What are the ages of the adults? How important is nightlife? What specific ships and itineraries are you considering? I know, I have probably created more questions than answers - but those are all considerations to be taken into account when making your decision. I would suggest you find a good cruise specialist travel agent to help you make the best possible choice for your group.