U.S. Government Sites:
 
Kids.gov
(An extensive site that educates children of varying ages about coins, currency, the history of money, as well as host of other monetary, consumer, and financial topics... all written for children.  Includes some video games too!)

U.S. Department of the Treasury, Bureau of Printing and Engraving
(Where paper money is made)
U.S. Department of the Treasury, U.S. Mint
(Where coins are made)

U.S. Department of the Treasury, U.S. Secret Service - Know your money page
(How to identify counterfeit coins, currency and other government notes and checks)
Useful Links:
 

CoinSuperstore.com - contents copyright 2003-2012 inclusive