Hello Laryn! Welcome to the Disney Parks Moms Panel.
Fairytale Dining at Cinderella's Royal Table and the
Happily Ever After Fireworks Extravaganza are definitely 2 activities that are not-to-be-missed during a day at Magic Kingdom.
Since Happily Ever After has a run time of just under 20 minutes and Cinderella's Royal Table is located within Cinderella Castle, I recommend that you watch the show from the hub located in front Cinderella Castle in its entirety and then make your way up for your
Advanced Dining Reservations at 8:30pm. While the crowd congestion is significant as Guests exit Magic Kingdom, the traffic is headed the other way down Main Street, U.S.A. and it should not take you more than 5 minutes to get to your destination.
Besides, in front of Cinderella Castle is the best location to watch Happily Ever After.
If you are concerned about timing, I have watched (and enjoyed) the fireworks from Fantasyland beside
Enchanted Tales with Belle and enjoyed the unique perspective from there. Cinderella's Royal Table is only 2 minutes away from there.
I believe you will be fine as far as being able to enjoy both Happily Ever After and Cinderella's Royal Table.
Have a Magical Night!
~Todd