Berick1234 is correct, it depends on the time of the year, and holiday season that dictates the amount of kids/children on NCL or any other cruise line. It would be extremely difficult to calculate an adults/kids ratio unless you had passenger data prior to each cruise. You would also have to secure the data of past cruises over a specified period of time.
I can tell you this. We sailed the NCL Star from Nov 10th to the 18th. On our cruise, there were 2100 passengers of which 1900 or so were adults while the remainder were children. On the following sailing which covered the Thanksgiving holiday we were told the ship was preparing for 2900 passengers that included 700+ children or kids. The cruise line and staff carefully checks the data up to 2 weeks prior to sailing and they know the exact amount of kids that will be on any one ship. They know the true adult/kids ratio, but the average passenger would not. On our latest cruise, we had private dinners with the Hotel Manager one night and the Food & Beverage Manager another night. During those dinners we learned alot on how the executive staff manages rooms, food, beverage requirements and procurement of food items.
So in essence, it would be widely expected to see higher kid counts on holiday cruises and summer time cruises. If you want to avoid a high kids to adult ratio, I would plan a cruise when kids are generally in school.
However, in most cases, the cruise line companies have well planned kids/teen programs and generally as an adult cruiser, you may not see the kids as often as you would suspect.
Personally, I would not deter any cruise from the fear that there may be too many kids on board. The ships are large and there is designated areas for the kids/teen programs, usually far away from any adult activites.
Book your cruise and ENJOY.
For us, it's the only way to vacation, kids or no kids on board.