Hello, Crystal!
Surprising your children with a trip to Walt Disney World is a fantastic idea...but it's so tough to keep the secret, isn't it? :-)
Regarding keeping both children happy while dining, book your Advanced Dining Reservations at any Walt Disney World restaurant that would appeal to your
15-year-old's palate, as there are children's menus available throughout WDW. The 5-year-old will be happy just about anywhere you go.
Before heading to WDW, take a look at the restaurant menus on a park-by-park basis. For example, once on WDW's website, click Magic Kingdom (under "Parks & Tickets") then select
"Dining." Click on each Table Service Restaurant to find menus. (If you happen to find a restaurant which does not have a menu posted, head over to the unofficial site allears.net, where you'll be sure to find unofficial menus, reviews and even photos, too.) Pick a restaurant or two in each park (in Epcot, it may be more like several!) which seems to have the food that's most appealing to you and plan your ADR for the day on which you plan to be in that park. (We look at park hours and Extra Magic Hours when determining where we'll be each day.) Don't forget to look at
Dining Options elsewhere, too, such as at WDW resort hotels as well as at Downtown Disney. Any location that is near the park you'll be in that day is a good choice, too.
Consider adding variety, too, by including a buffet or two, a character dining experience, a unique dining experience (such as
Coral Reef Restaurant at Epcot's The Seas with Nemo & Friends Pavilion, which allows Guests to gaze out to the park's 5.7 million gallon aquarium!) etc.
Send me more questions as you narrow down your choices; I'd be happy to help you decide! :-)