You can try online vocab builders, such as
http://www.sheppardsoftware.com/web_games_vocab.htm
and
free rice

Look under SAT Vocabulary.

or go to Amazon and buy 'vocab textbooks'

also - they have these great SAT prep books' with definitions of the difficult words in a margin at the bottom - classics.

I think a lot of the materials from 'Sparknotes' and other study aids would come in hand with these little ones, because they are simple and complicated at the same time. Bookstores around here have full bookshelfs of this stuff.

I think that this happened at our house, I have a terrific receptive vocab, but trained myself not to use 'big words' in High School, and kept along that track. Now I think that DS has missed that opportunity. He did learn the word 'Ambivalent' at age 3, and that come in very handy.

See if you can get your DH involved, because I think the best way for these kids to learn vocab, is to overhear it in use.

Smiles,
Grinity

Coaching available, at SchoolSuccessSolutions.com