I didn't try it myself, but the woman that was working with the agent as we waited was looking to book several different parties on a future cruise. I'm pretty sure they'll be more than happy to book as much business as they possibly can!
With six people in your party, you will have to split them into two rooms unless you book a Cat 1 or Cat 2. And since the first two passengers in each stateroom are billed at the adult rate (regardless of age), then each stateroom will be eligible for the credit. (When I rebooked on a previous cruise, I investigated two rooms for my family of five, and the DCL agent assured me that I would get the credit for each room.)
I believe that the OP will have to book a 3rd room for her inlaws...unless they want to stay with two of her kids. IDK if the kids are able to book a room with the discount on their own but if you do book a room for your inlaws, perhaps you could list one of the kids in the room with them & the child's CC number will give that room the discount & credit???
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