Howdy Mary!
I'm not aware of any current regular Meet 'n' Greets with Phineas & Ferb, but WDW makes changes periodically to the characters in the parks, so you should definitely check the park Times Guides when you visit, and also ask at guest services, just in case. I think the most likely park for Phineas & Ferb would be Hollywood Studio, but any park is possible. But, please don't promise anything - we certainly don't want disappointed 8 and 11 year old boys in the parks! :-)
Your boys may be pleased to see that these lovable guys WERE at Disneyland in December 2009 for the holiday parade (click
HERE to read about it on the Disney Blog). So, at least we all know that the characters exist in the parks, now we just have to be lucky enough to find them! ;-)
If you (or other readers) want to find more info about character locations, some resources I use include (please be aware that some information may be outdated - character greeting locations can change)...
- http://www.wdisneyw.co.uk/greet.html
- http://pages.prodigy.net/stevesoares/
And in the parks, you may find Disney's Mobile Magic application to be helpful to locate characters. Click HERE for more details from the Disney Blog.
Hope that helps. Have a great time!!
p.s. special thanks to @djdrew and @deeforDisney for helping me with some ideas to better answer this question