Twenty four days on a ship--even a wonderful ship like Zuiderdam--is a long time for adults--especially adults with two small children.
Several of your worries I believe are unfounded; for instance, the food should be excellent; and you will never feel "hearded"; cruise ships are use to accomodating lots of passengers and they do it quite efficiently--especially Holland America.
But on a cruise that long there will not be a lot of younger people with young children. I am all in favor of family vacations and taking children on cruises--it is a great opportunity to teach and practice the social graces; but I would urge you to rethink the length of your cruise. I would suggest trying 5-7 at first and see how it goes. Then, if everybody loves it, as I'm sure they will, increase the length gradually.