I agree with the others that reading is the very best way to improve vocabulary. I agree that classic literature probably uses a wider vocabulary, but it also uses some obscure and obsolete words. I think the key is to read widely, and look up unknown words. There is some marvelous contemporary literature that would also offer a rich vocabulary. But interest and quantity trumps quality. It's better for them to tear through fluffy books than for them to resentfully struggle through one classic.
While looking for contextual clues is an imporant skill, looking up precise definitions is also important. E-readers like Kindles (app and device) make this easy. All you have to do is click on a word and you can get the definition or conduct a web search.
For book selection, if you give us their ages and/or interests, we can give you more specific recommendations. I would say that regardless of their age, read to/with them. They'll be able to understand and engage with more complex material. Your pronunciation and inflection will help their comprehension and retention. While I know you're primarily concerned with vocabulary level, it needs to be secondary to the interest level. Pick books that you'll enjoy too.
While they've gotten a bad rap, you might consider basal readers. Whether it's the classic McGuffey readers or more contemporary reading books (available at secondhand bookstores or online), the vocabulary tends to be leveled, often including definitions of more advanced vocabulary and the stories can be engaging, but may be less appealing to teens.
As an aside, Just-So Stories by Rudyard Kipling offers beautiful (if unusual) language. They're basically folk tales for children that suggest imaginative reasons for why animals have their specific traits. In this case, I don't know that it's important to define each and every word. It's probably good for getting a lot of words from context so that you don't disrupt the flow of words.
Foreign language exposure is great, especially Romance languages which would relate to the Latin roots of words. If they can start to develop a sense of roots/prefixes/suffixes that will be tremendously helpful. You can help them break apart new words and analyze the parts. For example, "concise" con - with, cise - cutting. The more they practice, the more familiar they'll get with the patterns.
As much as I hate to admit it, the obnoxious poster earlier had a point that you as parent can model good behavior. As mentioned above, read to/with them, but also let them see you read by yourself for your own gratification. Be conscious and deliberate about your word choice using a rich and varied vocabulary. If you're using words like coax and concise in daily interactions they'll pick them up eventually.
Get them involved with words any way you can. Crossword puzzles and even seek-and-finds will get them thinking about words. Play games like Scrabble, Boggle, or Bananagrams. There are online versions as well. Play around with puns. Explore odd expressions when you encounter them, they can have fascinating origins.
Freerice is a website that offers leveled vocabulary quizzes and also provides rice to starving people around the world which might be motivating to your kids.
http://freerice.com/#/english-vocabulary/1549
Here are some other vocabulary related websites:
https://www.hoagiesgifted.org/language.htm