I don't think there's a Meet and Greet, but you can be sure to see them at "Phineas and Ferb's Rockin' Rollin' Dance Party", which is held several times a day near the entrance to Little Mermaid. Get there 15-20 minutes early for a front row spot. And do NOT stand on the side with your back to Little Mermaid - that's where the vehicle comes in and that area has to be clear, though once it arrives they let people stand there for the show.
They encourage audience members to come out and dance - the Fireside Girls interact with the dancers, and even P&F interact with dancers a little bit. The show is about 15 minutes long.
Not sure when your trip is - it's possible that after the summer season this show will only be on weekends.