Good luck with your sales pitch. You will find thousands of cruise fans to back you up here.
The cabins are comfortable. I did not feel claustrophobic in our Deck 1 cat 9 (oceanview) room. If felt like a small hotel room. Besides, how much time will you want to spend there, anyway. One of the nice things about
DCL is that rooms cat 10 or higher come with a split bath. That means a shower/tub and sink in one room and the stool and another sink in the other. Makes getting ready a breeze.
I didn't see anyone puking over the sides. LOL! My DD3 is prone to motion sickness and she didn't even feel the ship moving. I felt the motion mostly in the theaters and the dining rooms. I didn't have to take anything, only take a couple of deep breaths or move get some fresh air to feel better. Really not a problem at all. Let's put it this way, I didn't miss a meal.
But there are some very effective motion sickness remedies available, both natural and medical. A word of caution, make sure you try them out at home to test for any side effects. I tried Bonnine one time and I broke out in an itchy rash.
There are a lot of things for an 11 year old to do on board. If she makes friends easily, she will love the clubs. They plan a lot of great activities for the kids to enjoy. Gives Mom and Dad a break, too. If she would rather spend time with you, there are plenty of things to do together - shows, games, swimming, etc. Plus there are great ports of call to explore.
BTW - selling point for your DW. There is an adults only restaurant, adults only pool and adults only entertainment and seminars.
Castaway Cay is the most amazing place I have ever been.
If you like Disney, you will love DCL. The Disney touch at sea is unbeatable. Characters are very accessible.
Good luck!
Denae