Take the kids to Fort McHenry to learn about its role in the War of 1812.
The national anthem of the United States came about due to the fact that Francis Scott Key saw the flag flying over this fort after the Battle of Baltimore.
It would probably be a good idea to read the history of the fort to the kids before you go there for a visit. Their visit will most likely be more enjoyable if they understand what they are looking at. Wikipedia would be a good place to start for the history.