I think "hard" words is just fine. In fact we all have plenty of words in our passive vocabulary, so I don't see not using it as a problem at all; whether a given child will come across a given word is hard enough to predict that I wouldn't try! Even if the child forgets the word, no harm done; the process of thinking about the word is most of the point. I'd expect, out of a list of 10, some to get a "meh" reaction, some to be quite cool, and one or two to be useful.

FWIW my son enjoys vocabulary.com, too.

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