Laws on child passenger safety are passed by the states, rather than the Federal Government. The web site Safe Roads has a fact sheet on child passenger safety which includes a link "Booster Seat Laws by State": http://www.saferoads.org/issues/fs-child.htm
The Insurance Institute for Highway Safety also has a table comparing state laws on child passenger safety, which includes the amount of the fines for violating them: http://www.hwysafety.org/laws/state_laws/restrain.html
(Note: The above private-sector sites are not maintained by US DOT or any other US Government office. We take no responsibility for the content or accuracy of any non-US DOT web page, and offer the above link strictly as a courtesy, should you be interested.)
You may also want to contact your state DOT for more details on the laws in your state. Links to state Departments of Transportation and/or state government web sites are available at http://www.fhwa.dot.gov/webstate.htm
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