MEM,
I suggest that you go ahead and make reservations 120 days in advance (I think that's when you can). Soon thereafter, CALL the Grand Floridian Resort (not Disney Dining where you make the reservation), ask to be put in touch with someone directly responsible for the Pirate Cruise, and explain the situation.
I suspect things will be alright as long as they know what to expect. In the unlikely event that your DS can't do it, you can cancel one or both reservations.