Education about the operation of the U.S. Congress is typically rather poor, especially in elementary and middle schools. Parents can supplement what information their children receive in school, however.
One source of materials for supplementary education about government comes from the Hoiuse of Representatives: Kids in the House. Other resources are found at Congress For Kids, provided by the Dirksen Congressional Center.
Also see Ben’s Guide to Government, which is an okay reference for mid-age children, but doesn’t contain much adjustment to children other than to have cartoons of Benjamin Franklin in the margins.