I CRACKED THE MOBBY/BIKE CODE!!
First let me say, that this is my new favorite IMDB message board.
Now, for the answer to all our questions... maybe..
Taken from the IMDB "goofs" section of Radio Flyer:
Audio/visual unsynchronized: When Mike and Bobby are looking for Shane, their German Shepard dog, Mike runs through the gate to look for him. When Bobby finds him lying in pain, he calls for Mike, but the voice he says it in is not his voice, it is Mike's voice.
Maybe it's not a goof?! Maybe, just maybe, you crazy kids who think Bobby and Mike are the same kid really have something there. Anyway, I'm now a subscriber to this belief.