I understand your problem. My DD totally is into characters. The App is her new favorite thing and constantly is checking it to see what characters are out. You know your child better than anyone, but, here is what I do to help my DD. She looks at the App and then makes a list of the characters she wants to see. You could help with this and help guide this process. I kind of go through and get an idea of where they are and when they are usually there. I work that into our touring. I have limited the number of characters in the past because I find that is all that we could do - no other attractions. I have either done so many a day or 1 day devoted to characters. A lot of times, the characters have a cast member with them. I have approached them when we have been standing in line and the character needs to go backstage. They will let you know when and where to stand for when they come back. They will also let you know which character is taking their place. I have found they have been very accommodating and have even occasionally given my DD some pixie dust experiences.
Watch for the roaming characters as well. Mary Poppins and Bert are frequently walking through Fantasyland. Peter Pan is frequently by Pixie Hollow. Jack Sparrow is often by Pirates/Rivers of America.
I have found that meeting Mickey Mouse and Minnie Mouse around Main Street to be better than meeting them in Toon Town.
I also do character meals. Goofy's Kitchen for dinner or Plaza Inn for breakfast are our favorites. However, we have done PPH breakfast with Stitch in the past and had a total blast with Stitch. He was so interactive with my DD.
One experience that my DD will never forget that I would like to share. It was later in the day and we were leaving the park. There weren't a lot of people leaving at that time and it didn't seem overly crowded. I was walking ahead of my DD and suddenly I heard "Mom!" I turned around and here was Goofy walking right behind my DD. He was reaching his hands around in front of her face. It was kind of terrifying to my DD. However, I just looked at Goofy and said, "Goofy are you trying to follow us to the hotel?" and I laughed. Goofy had already realized that my DD was frightened and he had kind of backed off. He picked up on my reaction and come up to me and grabbed me by the arm and started walking along with me. I told my DD to come on the other side of Goofy and the 3 of us walked to the train station arm in arm.
My DD is totally in love and obsessed with Piglet. Piglet hasn't been out the last 3 years when we have been there. My DD keeps asking. The last time we stayed at the DLH, at check in, my DD was telling the CM checking us in that she "hoped Piglet was visiting." The CM left and came back with an autographed picture of Piglet. My DD was in heaven.
I hope you have a great trip!