Hi Sarah,
How exciting that you are planning your very first visit to the Walt Disney World Resort - you're going to have so much fun!
I, personally, feel that you will be cutting it a bit fine.
As a general rule, I would always suggest allowing at least an hour to enjoy a meal at any Table-Service Restaurant.
Coral Reef Restaurant is located in Future World at
Epcot and whilst you will be able to have a good view of
IllumiNations: Reflections of Earth from all around the World Showcase Lagoon, you will need to allow 5 minutes or so to walk from Coral Reef Restaurant to the World Showcase, as well as a few minutes to find a spot to watch IllumiNations: Reflections of Earth from, which isn't too crowded.
My best advice to you would be to arrive at Coral Reef Restaurant by no later than 8:00 PM on the day of your dining reservation and ask if it would be at all possible for you to be seated early. If possible, this should give you adequate time to enjoy your meal and Illuminations: Reflections of Earth during the same evening.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma