It's in the scene where JM is playing Cowboys and Indians with the boys. Peter tells JM that he is not is father and tries to run into the house. He is wearing a vest and a faded brown plaid shirt when he leans over to talk to JM (who is laying on the ground after being *shot*) when he turn to run the vest is not there and he is wearing a white muslin or linen shirt.

Later in the same scene he is grabbed by the vest by George who yells "I've got him." A scuffle breaks out and the vest and shirt changes back and forth between outfits. When he finally gets away and runs into the house he is wearing the white shirt.