I'm sorry to tell you that as of today, the Phineas and Ferb gang are not part of the regular meet-and-greet line up at Walt Disney World. They did make a special appearance greeting guests as part of the Disney marathon festivities earlier this month, but they're not normally in the theme parks.
However, the characters available to meet at WDW are
constantly changing and there is always a possibility that Phineas,
Ferb, and their friends may be added to the regular park greetings at some point in the future.
To make sure that you're not missing anything, I would go to the Guest Relations office at any of the theme parks;
these are noted on the park maps. Ask them to call the "character
hotline" for you. The character hotline will be able to tell you if
any particular character will be in the parks on a particular day, and
if so, where and when the character will be there.
I'll keep my fingers crossed that your children get to see their favorites.