In my mind, the moment that Bruno changes his clothes and hops the fence is when a point of innocence is lost. I say this because it is at this point that Bruno has done a couple of things that represent a narrative of experience as opposed to innocence. The first would be the disobedience of his father's instructions to not go to certain parts of the camp. The edict of "Out Of Bounds At All Times And No Exceptions" is completely discarded when Bruno crosses the fence. While it is valid that he had already tested out this maxim by actually approaching the fence, the notion of crossing it, of violating the demarcation of what is accepted and forbidden, represents a moment of experience supplanting innocence. Along these lines, the changing of clothes might be a symbolic moment where innocence becomes replaced with experience. Bruno empathizes with Shmuel enough to be able to move his frame of reference into his. In its purest of terms, this is no longer innocent, complete in one's own self- absorption. Rather, it is an act of maturation and insight for Bruno to not only walk in another's shoes, but wear their clothes and absorb their experience. There is probably a loss of innocence at the moment that Bruno recognizes something being wrong as he and Shmuel are herded into the gas chambers. To probe into Bruno's mind at that moment would be a loss of innocence as he either realizes the dire consequences of what is to be or fully grasps the faith in his friendship as a way to ward off the horror of what is to be.