Since kids only get one week, and sometimes one week and a day, such as Easter Monday, for spring break, you won't see many kids on a 10-day cruise. That would require too much time off for school, particularly since there's the requirement of flying in a day or two in advance on top of the time off for the actual cruise. I recently asked this question elsewhere and the consensus for a week-long cruise was that there was anywhere from 200-400 kids on a spring break cruise with a ship of approximately 2,700 passengers.