Hi there, Mark!
Cinderella's Royal Table definitely will require some advanced planning; it is one of the more popular table service restaurants at Magic Kingdom.
First things first. You will need to call (407) WDW-DINE to see if in fact two more people can be added to your reservation. If, at the time of our reservation, the restaurant is at capacity, you will need to figure out an alternate time.
And yes, as you note, this restaurant does require pre-payment.
There are no refunds due to no-shows. You must cancel 48 hours in advance or forfeit the prepayment. Call (407) WDW-DINE (939-3463) or (407) WDW-CNCL to cancel.
Have a wonderful time!
See you later, celebrator,
-Deborah B.