Hi Chris.
I teach children ages 7 to 9 and use flashcards to
a)introduce new vocabulary (a great site is
www.esl-kids where you can print out flashcards with or without words. With the kids, I choose flashcards w/out words so as not to confuse them). Hold up the flashcard, elicit the word, chorus chant the new vocabulary.
b)review vocabulary. At the beginning of each lesson, I play a
line game where I line up the class in 2 groups and go over some of the words they've learned. Hold up a flashcard, elicit the word, the student who gets it right gets to sit down, the other student goes back to the end of the line etc...
c) the slam game that Eric mentioned is ALWAYS a big hit with the kids
As to the question whether flashcards are also useful to teach sentences, you can do both. When I taught action words, I used the flashcards depicting various actions/activities to build sentences, i.e. "what are you doing?" " I am ....-ing". So instead of just learning the word, the kids learned how to make sentences.
d) when teaching shapes, I made smaller versions of the flashcards and hid them in the classroom. Each student had to find 2 different ones and describe them to the class, e.g. "I have a star and a circle". They got to keep the cards which they loved.