Sorry, but I completely
think that symbolism is a VERY important part of good literature. As someone who writes, reads, and studies literary fiction, I can assure you that symbolism is deliberate and intended and yes, authors (even Mark Twain) want, NEED you to see it. If we don't teach kids how to see abstraction (and little is more abstract than that, other than math, perhaps) then they miss HUGE parts of great novels and films.
Having said that, this is a developmental skill and kids, even the gifted ones, are ready for it at different times.
Symbolism is such a part of art, film, and literature. We do our kids a great disservice if we denigrate a large part of what makes these things beautiful and meaningful. By teaching metaphor/symbolism, we really open up literature for our kids.