There are a lot of nice campgrounds in Lancaster - make reservations now. Old Mill Stream Campground is next to Dutch Wonderland - an amusement park for younger kids - perfect for a ten year old and the 14 year old is just on the edge but should also enjoy it. We stay in Old Mill Stream a lot and have not been bothered by any noise from the road which is some distance from the actual camping sites and a blocked by two buildings.
Gettysburg as has been said is a tough reservation this year but you should be able to get a site. The Battle of Gettysburg is difficult to understand because it took place over three days and is spread over a lot of ground. To best understand the battle and make the tour of the battlefield much more enjoyable rent or buy the movie, Gettysburg - starring Jeff Daniels. You can usually find this DVD for $5 now. Watch it with your family and you will get a real understanding about the battle and what took place plus you will see the sites you saw in the movie when you get there and have a much greater appreciation for them and a whole lot less confusion about what you are seeing. The movie is long but very entertaining and very well acted with a big cast. It was filmed in the town and in and around the National Park.
Gettysburg DVD