As an eNotes Educator who is currently the mother of a nine-year-old girl, I feel especially equipped to answer this question. No, I don't think Bruno is too naive. My child continues to hear words wrong (one example is "corder" for "quarter"). So Bruno's misunderstandings of "Auschwitz" as "Out-With" and "the Führer" as "the Fury" are completely believable. Remember that Bruno's innocence is always infused with wisdom. Bruno questions almost everything the Nazi regime stands for. Here is a quotation that is a perfect example:
What exactly was the difference? He wondered to himself. And who decided which people wore the striped pajamas and which people wore the uniforms?
Bruno's simple questioning of the Nazi belief here shows his lack of naivete. Further, Bruno proves he is not naive further through his relationship with Shmuel. Bruno simply rejects the idea that a Jewish boy like Shmuel is not "a person" such as Bruno's father would have Bruno believe.
Very slowly [Bruno] turned his head back to look at Shmuel, who wasn't crying anymore, merely staring at the floor and looking as if he was trying to convince his soul not to live inside his tiny body anymore, but to slip away and sail to the door and rise up into the sky, gliding through the clouds until it was very far away.
Bruno's observation here is far from naive. It proves that Bruno sees Shmuel as a person with a soul: a person who wishes to escape, even if that escape is only possible in his mind.