Off the subject, but a comment about your plans for Gettysburg. I used to live up there. It is about 1-1/2 hrs from DC so plan on that drive, but a very pleasant one. I forget how you get over to Rt. 15, but you want to go up past Frederick, Maryland. You will pass through the Catocin (sp?) mountains/Thurmont MD, near where Camp David is located. Beautiful area. If your kids like to be in some natural areas, there is a great park and lake in Thurmont (Cunningham Falls State park). It is just a little off the main road, but close enough for a good spot to stop, picnic, and enjoy the mountain scenery.
Another good thing to do is to see the movie "Gettysburg" first. It will bring it really "alive" when you walk the battlefield. Lots of good little shops, places to eat, etc. Be prepared for narrow streets and traffic, however. It is a tiny town, but very alive with the history and feeling of Civil War times. Enjoy your trip!