Welcome back, Bill! Since you've already sailed with Disney Cruise Line, you are officially a Castaway Club member, which means that you should be able to log into your reservation with Disney Cruise Line 90 days prior to sailing to begin booking those fun extras.
When you begin the
online check-in process, you'll see a box called "My Cruise Activities". There you can select Port Adventures and make reservations for Adult Exclusive dining. Your regular dinners do not require reservations, as you'll already be assigned either Main Dining or Second Dining, based on your preferences or availability when you booked this cruise.
Disney Cruise Line is certainly wheelchair-friendly, as they want all guests to be able to experience the fullness of a Disney Cruise Line vacation, no matter their mobility. However, if your parents require more specific accommodations, you might consider booking a
handicap accessible stateroom. The agents at Disney Cruise Line at 800-951-3532 can chat with you about the availability of those staterooms on your exact sail date. Disney does offer a few sand wheel chairs at Castaway Cay, free of charge, on a first-come, first served basis, near the Pelican Plunge tram stop and Scuttle's Cove tram stop. Those are wonderful for guests who require wheelchairs to get around the island, as they make it much easier to navigate, even on the beach itself!
I hope those details help as you continue planning, Bill!