Cruises tend to sail "sold-out", with all available cabins filled. Since Disney only has 2 ships in their fleet, they probably have even less chance of sailing with empty cabins. Unlike other lines, hordes of passengers travel with very young children - the school calendar doesn't apply when you're age 4 or 5 <g>. Many will take the school-age kids out of school for a week, too.
Disney will limit the # of kids on board, but it's a very high limit! Basically they have a certain capacity in the kids clubs and won't take bookings for more kids than they can accomodate. Those of us traveling without kids generally don't have trouble avoiding kids, who tend to be in the most crowded/congested areas (such as near the characters, at the family or Mickey pools, etc).