Hello! Being that the ship is so large, you hardly feel the motion to get sick. Plus he is going to be so occupied with all the things going on the ship, I feel the "boat sickness" will not even occur. As far as what room is better to avoid feeling any access motion that may contribute to any motion sickness, I say book a room that is higher up. We were on the very bottom level of the boat, and could feel lots of swaying. (though it rocked me to sleep!) But, by the time you get to your cabin at night, you are sooooo exhausted, these things shouldn't really bother you much.
Nassau I think was like 10am to 5pm? You have to take a short drive to atlantis. We did the atlantis, it was neat and all. The areas they let you in are the casino and a walking tour of the aquarium. Really wasn't what I had expected. Atlantis was hyped up alot from different advertisements, in my opinion....I would never go back to atlantis to stay. But since your kids are a bit older (my daughter was 4) I would probably recommend the dolphins. They would get alot more out of that experience.
If this is your first cruise, you may want to go through at travel agent. They can give lots of tips on how to get ready for your cruise, and can sometimes search for discounts you didn't know existed.
There was nothing fancy about the disney cruise. We dressed up one night for what we thought was suppose to be formal night. Well, we were waaaaayy over dressed!!!
I felt sooo uncomfortable in my fancy black dress and heals. oh well, got some nice pictures though! If you plan to dine at all in the boat's private restraunt, that is the only place you will need to dress up in. Otherwise, casual in the main restraunts.
have a great cruise!! book it!!!