Our experience is, that you find on the longer cruises, 11 nights or longer, less children then on the short cruises.
And you have to watch the holiday seasons.
We have been on a 14-night-cruise with the Zenith in february this year and there were less then 10 kids on board. And for example, they were not allowed to take the elevators without their parents.
But they told us, that they also have up to 300 children on some shorter cruises and that on a Celebrity ship. I didnt expect that. I thought people with kids would prefer going on RCCL, Carnival or Disney ships.