You are indeed travelling over the half-term holidays and also Halloween so expect lots of children. We have just returned from the mini cruise to cork and were told that there were 800 children on board (2 of them ours). This in no way spoilt our holiday and I never saw any bad behaviour. Although we tended to watch the evening show and be back in our suite by midnight. If you do have any problems COMPLAIN COMPLAIN COMPLAIN and don't stop until you get something done, when they realise that people aren't just going to put up with it they'll behave. Don't worry you'll have a brilliant time. It is a beautiful ship.
DH was also on the 4n cruise to Cork and he too said that he did not witness one item of bad behavoir on the cruise even though there were hundreds of the little cherubs on board! I guess you never can tell what 'pack instinct' they will develop when you get a gang of them together.
I wouldn't worry too much! Half term is only a week, and children on the cruise would also need to have time off school which will limit the number on board. As a teacher, it has been my experience that parents who pull their children out of school for a holiday generally leave it until the summer term rather than so close to the beginning of the academic year. There will obviously be some sailing on this cruise but hopefully they will not spoil your enjoyment too much!