In a country on the verge of personal and national financial collapse, I don't think we can begin teaching our kids about money at too early an age. Most experts advise giving kids their own piggy bank at about age 5, but some 4 year olds are ready to learn about money, too.
That's one reason I was so pleased to find a great little book called The ABCs of Handling Money God's Way by Howard and Bev Dayton. My 4 1/2 year old really likes this book, and I've already noticed differences in her behavior regarding money. For example, she's more willing to work hard, and she's saving money she earns from extra chores for a toy she wants.
Although the book does teach a tiny bit about different types of coins, the main focus is on a biblical perspective of money, including giving, saving, honesty, staying out of debt, working hard, and good stewardship. You can read my full-length review of the book over at Christian Children's Book Review. I really can't recommend this book enough; in my opinion, it belongs in every Christian household with children 4 - 7.
If your kids are a bit older, about 8 - 12, you might also consider Howard and Bev Dayton's The Secret of Handling Money God's Way. I've not read it myself, but my friend Tanya Dennis, who has great taste in kids' books, highly recommends it.