finally figured it out **SPOILERS**
**SPOILERS AHEAD**
I couldn't figure out what happened the first time I watched it but I finally rewatched it tonight and YES!!! it all makes sense. The director was very crafty, however, and besides Hanks discussion at the beginning, where he tells his sons, "History is in the mind of the teller, and the truth is in the telling", you have to wait until the last 15 minutes or so and if you pay very careful attention you can see that Bobby was created in Mike's mind to handle the abuse.
They both get beaten up at the same time and suffer same side black eyes, they both have a stomachache at lunch in school with a bad premonition about their dog; right after Bobby's "flight" to freedom, his stepfather is handcuffed by the policeman who says to him, "You're done!" and Mike hugs his mother and says that Bobby is safe now. He no longer needs "Bobby" because the king is finally going to rot in jail. When Hanks talks about growing up and forgetting the magic of childhood imagination, (which includes the "ability to fly") he mentions that whenever he forgets, he gets another "postcard" to remind him.
"Bobby" wanted to fly, and it is Mike who becomes a pilot as an adult. Sampson was supposed to be on the Radio Flyer, yet they still have him. Because the Radio Flyer flying was in his imagination. It was a metaphor for escape. He says his mother never understood, because "being grown up, she could never recapture the frame of mind needed to really understand."
As an adult he has a picture on the wall of his mother and his dog, and a little boy (which is a picture of him as a child). There aren't two boys in the photo, and there are no adult pictures of "Bobby" who doesn't exist.
Hanks states at the end that keeping a promise is very important. And he kept the promise to himself--he promised he would never tell anyone that he was getting beaten by the stepfather. Not even his own kids.
It was fascinating watching this story from this perspective. I have new respect for this film.
I have always been proud of America.