I agree with Miss Manners here. Expecting all children to believe that all adults are perfect is a bad idea. Children still should be expected to be polite and mannerly, even to adults they don't personally respect. We are not discussing whether the library lady teaches her kids that all adults are perfect. We are talking about politeness and manners.
However, Miss Manners also writes:
It is neither wise nor reasonable to expect children to think that adults
are, by definition, smart, right, or admirable. Such an attempt will quickly
lead a child of even average powers of observation to the conclusion
that at least one adult, the one who makes this claim, is either dim-witted
or mendacious...A child who is never allowed to betray the belief that any
adult has done anything wrong is one who will quickly lose interest in the
idea of being among such deluded fools.