I was last on a
DCL cruise when I was 10, and my two younger brothers were 8 and 6, just bumped two years ahead of your kids.
I believe that my family and relatives (there were 10 of us going) booked one category 3 room and one category 4 room with a family in each room (2 adults, 3 kids in each family). My family was in the category 4 room, and it was great. It was a great size, and there were enough spaces for us to sleep in (my brothers got the bunk bed, while I got the rollaway, but that's what you get for being the oldest and the only girl). It was also great because my then-8 year old brother was deathly afraid of heights, water, and boats and being on a cruise didn't help. So he did have some nightmares, but our parents were just a hop, skip and jump away from the bunk beds and could help him in a flash.
But if you do choose two category 9s, 10s, or 11s, make sure to get connecting rooms so as your little ones won't have to go in the hall and knock on your door or remember a room key in case of "accidents" or nightmares.
Also, our cruise was in April, during Spring Break time. Well, the general time anyways because there are different times for different schools. We went on the 7 day land-sea cruise, and the parks were extremely crowded. We waited in line for lines that we could wait in line for half the time at
Disneyland on the same day (we've been to Disneyland on Spring break, and it was only half as crowded). The cruise and ports were crowded too, especially Castaway Cay, even though it was only our cruise. My suggestion is to go on a 4 day cruise on inservice days special to your district- mine has about 30 of those. That way, your kids only miss one day of school. And, as a word of advice, go in March or October, or another month that conflicts with no long breaks and isn't super cold or in hurricane season.
Hope this helps!