Belique is exactly correct, IMO. If you figure it all out, I'd be willing to bet you will be money ahead to take your family on the cruise, and forget the extra children. Young people, (as well as us old timers) will always find new friends on a cruise ship, and probably have a much better time too. Sit down and push the pencil, and be realistic about the cost of the food. (You don't need lots of shore excursions either to have a great time on a cruise)
That's my opinion!
See you aboard,