Here are some ways to help your child build vocabulary as he or she reads:

• Ask your child, while reading, to place a sticky note on each page of a book that has unfamiliar words. One can also just write the unfamiliar words, and the page numbers they occur upon, in a notebook. You can look them up in a dictionary later, and refer back to the book.
• Play dictionary games with your child. Take turns reading unfamiliar words to each other, and try to guess their definitions just from the sound of the word. Try to stump each other!
• Play a description game with your child. Look at a place or object and say a single word that describes or relates to it. Then your child has to say a different word that describes it, and you take turns until one of you can't think of a word. If you're looking at a tree, you might say "green," and your child might say "tall." Pretty soon, though, you will run out of easy words and have to say "evergreen" and "photosynthesis." This gets both of you to use words that you don't normally use.