Ahoy, Monica!
I am happy to hear that you will be sailing with Captain Mickey and his gang in December! Your family will most certainly have a WONDERful time on the Disney Wonder.
Planning an adults-only meal is one of the best indulgences on any Disney Cruise Line vacation. Even if your
Palo dinner falls on the ship's optional formal night, you are certainly welcome to visit the atrium with your family to take photos while you are all dressed up. Be sure to check what hours the ship's photographers are available that evening, as they are not in the atrium during each dinner time.
Depending on your dining time, you can visit the main dining room that you are scheduled to dine in and sit with your children for a bit. I have found that the dining room servers are usually willing to have just your children come to dinner, and imagine how cool that must be for the kids! Our daughters have gone to meals alone before and they just love it!
After looking at your cruise itinerary, I would guess that your sixth night is indeed the optional semi-formal night. The formal and semi-formal nights usually take place on days at sea, and are almost never the first or last night of a Disney Cruise Line sailing. Should you want to reschedule your dinner at Palo, you can do this either online or once you board the Disney Wonder in December.
Please come back and visit us again here at the Disney Parks Moms Panel with any more questions before you set sail!
Happy Cruising, Tracy V.