I understand you question was..."I am traveling to Disney World next weekend. And because we just booked
our trip I was unable to get reservations for two adults at
Cinderella's Royal Table. Do park guests ever cancel, and do they allow
for people to take their spots?"
Brigitta gave you some great advice (of course!) to continue trying, as people do cancel. In fact, since this restaurant has a 24-hour cancellation policy (you lose your deposit, if you don't cancel within 24-hours), it is not uncommon for people to cancel their reservations a day or two in advance. You
will need to call Disney Dining (407-WDW-DINE) to make reservations, as this is the only way to make reservations for Cinderella's Royal Table. Even if you are at the restaurant in the Magic Kingdom, they will not be able to help you, but instead will likely ask you to call Disney Dining.
This is a VERY popular restaurant, which often fills up within minutes 180 days in advance, so please be aware that you may not be successful in your quest. If I were you, I would definitely look for some back-up alternatives as well, just in case. The Princess meals at Epcot (
Akershus) and the Grand Floridian (
1900 Park Fare)are other alternatives that I would highly recommend, and typically have a bit better availability.