Visited Gettysburg a few years back when my youngest was 9.
As one buff to another, its not a place they will enjoy, without them being interested in the topic. Nor will you get the time to enjoy the site.
Taking my young ones to national parks works really well, or to places that are reenactments or living history. But outside of that, places like Gettysburg are essentially museums.
You are so close, you'll be able to take them again when they are older.
Leave the kids with relatives and just the two of you go for 2-3 days. Otherwise its really just a trip for the kids, that happens to be at Gettysburg.