Hi Jodi-Ann, thanks for stopping by with your fun question.
I have great news, recently Phineas and Ferb can be found for a meet and greet at Disney's Hollywood Studios around Mama Melrose's Ristorante Italiano Courtyard. Keep in mind that these meet and greets and character availability can vary day to day. However, the best way to know when are where to meet your favorite characters is to get the
My Disney Experience mobile app. In fact, that is what I used during my recent trip in June and I actually just used it to find out where Phineas and Ferb were today to get a reference point for you. This app is an amazing tool that can also be used to get official up-to-the-minute wait times for attractions as well as park hours and other great features.
The
Phineas and Ferb: Agent P's World Showcase Adventure in Epcot is tons of fun and a great way for your kids to enjoy Epcot while the adults can see it for their own enjoyment. My oldest two kids are the same age as your two and they had a blast doing it. The great thing about it is once you finish one mission, you can either do another one or quit. They aren't too long (about 30-45 minutes each) that you can't finish at least one or two, but if you were in Epcot the entire day and depending on your pace through the countries, you could ultimately do 3-4 missions. My kids only did a couple, but it's not to say that it can't be done. Since they are supposed to be on a secret mission, my kids enjoyed going off into a secluded area to hear the message. This also made it easier for them to hear the entire message without the distraction or noise of the crowds around them. They really got into it.
I hope your kids have an awesome time!
~Allison L