Wow, I really like that idea. Put the nanny and the kids in the cat 11, and you and husband take the cat 9 across the hall. That could actually be cheaper than 2 adjoining cat 9s...again, depending on the ages of the kids.
Just be fair to everyone...spell out ahead of time how much nanny will be expected to work and when she will have free time. What salary will she get, or is the cruise alone her payment? How often will kids be expected/allowed to be in the kid programs, and will she be the "beeper contact" when they are in the kid areas? What happens in the event of a sick child or whatever emergencies might occur? Having taken my nanny on trips when DD was younger, it really helps to make these things clear in advance....even if it is a nanny you have at home also.
Seriously, the difference between a 9 and an 11 is $100 or more depending on cruise dates. Since this difference would apply to the first 2 passengers in the room, be sure to book the oldest child in this room That could be a $200 savings or more...which almost covers the cost of a young child! If you have an under 3, you would also save the cost of using Flounder's Reef!!!! This is a really great idea!!!