I am not sure that there is an effective innoculation until the child has acquired the intellectual/emotional ability/level/sophistication to comprehend/internalize that you cannot assume the truth/correction of anything you hear/read. While I cannot pinpoint when it occurred, it was obvious to me that DS10 was over that particular hurdle at age 5. Fortunately when he was very young, I don't think that he particularly cared or paid attention to strangers' comments. These days, I don't need to say a word as while DS remains polite, he does get this look (not as rude/obvious as eye-rolling) that confirms that he recognizes the hyperbole.
I think you're on the mark. DS seems sensitive to others' comments--he's visibly put off by the excessive attention and fawning over what is treated as quotidian at home--so I'll need to carve out a roadmap for those years from 2-5.
ETA: I don't mean for any of this to be construed as my not recognizing or praising DS' efforts/achievements. Far from it. I'm a cheerleader for whatever is new, exciting, and challenging. But I--and strangers and family-- don't need to hoot like a ninny every time DS reads a sign, does math, or uses a tricky word appropriately. He knows he can, we know he can, and DH and I show our pride by following his lead encouragingly. I think this is probably implicitly understood, so I'm likely being redundant.
