Didn't know about the "2 kid problem."
Definitely hit the Yorktown. Depending on how old they are, your kids will love it. An old WWII aircraft carrier and WWII submarine are spectacular sights to behold when you're young. I've been maybe 3 times. All before the age of 10(I'm 31 now) and I still remember each trip like it was yesterday.
http://www.patriotspoint.org/
More for the kiddies:
Charles Towne Landing:
http://www.charlestowne.org/index.shtml ....landed in 1670. If that ain't history, I don't know what is. It's aimed towards children.
Charleston Kid's Museum next to the Visitor's Center
http://www.explorecml.org/
Charleston Museum
http://www.charlestonmuseum.org/topic.asp?id=1
Charleston Aquarium
http://www.scaquarium.org/
The IMAX theater is right next to the Aquarium.
The Battery, Ft. Moultrie, or Ft. Sumter...birth of the Northern War of Aggression.
Carriage tour...if you really want to.
Walking tour if you're in the mood. Some of the ghost tours are neat.
The Palmetto tree on our state flag derives from the Revolutionary War...Ft. Sullivan(Sullivans Island) was constructed of Palmetto trees and when the cannonballs hit the fort, they'd bounce off.

Hence the Palmetto tree.
The Crescent "moon" derives from the hats that our defending Army was wearing during the first invasion. The Cresent "moon" shape was embroidered on each hat. From what I've heard, it was a derogatory insult towards the british army. The Crescent "moon" looks iike the armor that british officers wore around their necks and the SC soldiers were mocking them by wearing the same shape on their hats...........................
I could go on...and on......and on...............

i love SC