Hi Elizabeth!
Who can blame any 6 year old for wanting to meet Phineas and Ferb and the incomparable Agent P?
The best place to find them is in Disney's Hollywood Studios Theme Park. Check the Times Guide, which is available as you enter the Park, for locations and times in which you can locate them throughout your day.
Also, while in EPCOT, you may be interested in participating in
Agent P's World Showcase Adventure! This new, interactive experience allows you to play the part of a "Secret Agent", searching for clues throughout the World Showcase! It's completely free to take part in, with your entrance to EPCOT, of course!
Have a wonderful time at Walt Disney World - and good luck in preventing Dr. Doofenshmirtz from taking over the entire Tri-state area!!!