Hi Karen,
Walt Disney World specializes in pleasing guests of all ages. A ten year age difference should be no problem during your vacation this coming May. Let me suggest some attractions you should be sure to see in each of the four Disney theme parks:
Disney's Hollywood Studios: Everyone will love The Great Movie Ride, Fantasmic!, and Toy Story Mania! Your teen will love the Twilight Zone Tower of Terror, the Rock 'N Roller Coaster, and the updated Star Tours ride. Be sure your daughter sees Disney Junior - Live on Stage! and Voyage of the Little Mermaid.
Epcot: You will all enjoy visiting the gorgeous pavilions along World Showcase. If your son is an adrenaline junkie, he'll adore Test Track and Mission Space. Be sure to enjoy Illuminations as a family to cap off your evening.
Disney's Animal Kingdom: I suggest you all start your day by enjoying Kilimanjaro Safaris. The shows at this park (Finding Nemo - The Musical, Flights of Wonder, and Festival of the Lion King) please guests of all ages. Save Expedition Everest and Dinosaur for the oldest and bravest members of your family.
Magic Kingdom: You can all ride and enjoy most of the attractions in this park. Space Mountain, Big Thunder Mountain, and Splash Mountain, however, are more appropriate for older children.
As for the Disney Dining Plan, I'm a huge fan. There's no more cost effective or hassle-free way to feed a family while on vacation. If there's anything you'd like to know about the Disney Dining Plans, feel free to post a follow-up question
here.
Best wishes for a magical vacation!
Jill