Hi Amy!
Thank you for bringing your question to the Disney Parks Moms Panel!
This is a great question. You can theoretically do both, but since you are paying (quite a bit) extra for
Mickey's Not-So-Scary Halloween Party and all of the unique entertainment that it provides, I would not want to do both on the same evening.
The party festivities will begin at 7:00 p.m. If you dine at
Cinderella's Royal Table at 6:45, you probably won't be finished until after 8:00 p.m. With the special parade, fireworks, characters, trick-or-treating and shows, you wouldn't be able to experience all that the party has to offer. The parade and fireworks are definitely not to be missed!
If it were me, I would try to find another time to dine there. I understand it's a hard reservation to get, so you will have to decide how important having a meal there is if no other times work for you. That would just be my suggestion!
I hope this helps! Please let me know if you have any other questions.