Hi Twins4mimi
I also have a set of twins ( girls ) they are almost 10 and my son is 11. Since yours are much younger you probably won't be doing rocknroller coaster, but the character shots are priceless, because they are not just posed pictures. I have one of Minnie and my daughter face to face and Minnie is patting her cheek. A couple of good places to find the characters are at EPCOT first thing at rope drop when everyone else is heading to Soarin, TT and MS. There is a meet and greet just to the right of ice station cool. Mickey, Minnie, Pluto, Goofy and Donald are there.
Also, at Camp Minnie Mickey in Animal Kingdom there is another meet n greet and the same characters are in safari costumes. Go early for shorter lines. Just make sure you are finished before Festival of the Lion King lets out.
Oh, if you like Figment, you can find him to the right of the exit at Journey into Imagination. It's kind of out of the way and there was no one there when we went. There was not a photopass Photographer there though, so make sure you have your own camera.