Hi, Jana! Epcot is the only one of the four WDW theme parks that is like two distinctly different theme parks rolled into one. Along those lines, Epcot's Future World and World Showcase usually have two different opening and closing times as well. Although times are always subject to change without notice, I've included Epcot's online schedule
here for your convenience. You'll notice that on August 18 Future World (which includes The Living Seas Pavilion where The Coral Reef Restaurant is located) closes at 7 p.m. Therefore, the pavilions and attractions of Future World will more than likely be closed by the time you finish dinner at the Coral Reef. It may be around 8 p.m. by the time you enter World Showcase after dinner.
In that case, I would recommend grabbing a drink and/or dessert from one of the quick-service restaurants or kiosks located around World Showcase and finding your spot close to the lagoon for a great view of Illuminations at 9 p.m. If you have impatient ones in your traveling party, send them on to do some sight-seeing and window shopping while someone else reserves a park bench. You'll be glad to have somewhere nice to sit once the show begins, and having a nice cool drink or a yummy dessert sweetens the wait time. I hope this helps you plan! Have a wonderful day and a magical vacation!