Our kids are 8 and 12 and we have been on Carnival and Disney with our kids. Our Disney cruise was fun - the shows were made for the whole family, but Disney was not our favorite. I felt like the cost was not worth the experience. My kids were not interested in having pictures taken with the characters in the lobby - which is a lot of fun for many families. Castaway Key was nice, but I am happy on any beach. The table service and food was the LEAST favorite of all of the cruises I have taken (Carnival/Royal Caribbean/Disney)We even complained but it did not get much better. The kids program is from 9 am to 9 pm (or later - can't remember for sure). We went to see shows together on Disney and they were wonderful! We also saw a 3-D movie on Disney.
Carnival has the big water slides which both of our kids love. My kids said they enjoyed the Carnival cruise "way better". We spent most of our time in port or at the ship's pool. I hope that helps a little.
2008 Navigator of the Seas
2008 Carnival Inspiration
2009 Disney Wonder
2010 Carnival Conquest (June)