Hi Kelly! It's so relaxing to watch the fireworks while you're eating dinner. The best spot to have a table-service meal at Epcot while watching Illuminations: Reflections of Earth is the
Rose & Crown. You can make advance reservations at this restaurant in the United Kingdom pavilion online or by calling 407-WDW-DINE. Unfortunately, you won't be guaranteed one of the outdoor patio tables. The best way to snag one is to make your reservation one and a half hours before Illuminations. When you arrive at the Rose & Crown, tell the castmember that you'd like to have an outside table so you can view the fireworks during your meal, and be willing to wait a bit for a table. If you aren't seated on the patio, you can still walk outside to the patio to view Illuminations during your meal.
The Cantina de San Angel in the Mexico pavilion is a quick-service eatery where you can dine at outside tables while watching Illuminations, but you can't make advance reservations here. The adjacent table-service restaurant
La Hacienda de San Angel has huge windows overlooking the World Showcase lagoon and has a nice view of Illuminations if you're seated at one of those window-side tables. You can make advance reservations here, but like Rose & Crown, you're not guaranteed a table with a view, so arrive early and make a request for one.
Hope you have a magical vacation in May! Come visit me again at the Moms Panel if you have more questions! I'm happy to help!