I've not cruised on
DCL before, but hear they are fantastic. We've done several cruises on
Royal Caribbean and love it

.
There is so much to see and do on the ships. If you are concerned at all about seasickness, then I would try a 4-5 day cruise, otherwise the longer the better

. We've done the 7 day cruises and as an adult I think I would be better off with the 8-10 day cruise, however with kids 7 is probably the best.
One of the things I look at is how many stops the itinerary offers. Because I like to spend time on the ship exploring, I prefer a smaller itinerary of stops. Usually on a 7 night cruise, you have 4 stops (some offer 3). This would be the max I would do (prefer the 3 stop). This gives some recovery time on the ship after exploring the islands. Most stops are pretty close together, so something to consider with a little one.
I would also recommend the Western itinerary. We found that their are a larger variety of activities, versus the eastern which is geared more towards shopping (although there are some activities as well). It all depends on what you and your family like to do.
If you visit cruisecritic.com, they have a cruise discussion board which is extremely helpful, same as Disboards.
ENJOY
