As for plenilune and argent, they are beautiful words before they are understood – I wish I could have the pleasure of meeting them for the first time again! — and how is one to know them till one does meet them? And surely the first meeting should be in a living context, and not in a dictionary, like dried flowers in a hortus siccus!
Children are not a class or kind, they are a heterogeneous collection of immature persons, varying, as persons do, in their reach, and in their ability to extend it when stimulated. As soon as you limit your vocabulary to what you suppose to be within their reach, you in fact simply cut off the gifted ones from the chance of extending it.
Letters of J.R.R. Tolkien, #234, 22 November 1961 (via the Oxford Inklings blog)
I like his notion that meeting words “in the wild” is the best way. Oddly enough, I think it is close-minded adults who need to be addressed with a trimmed down vocabulary. Use a big word on them and they might dismiss you as a snob. Use too many in a row and their eyes will glaze over. It is assumed that children will do this for certain and perhaps sometimes they do, but the eager ones will keep listening and derive it’s meaning from context as soon as possible.