Hmmm, I kind of disagree, Gettysburg is almost more a park than a museum.
If I had kids I'd spend a day at the campground swimming, biking, then a half day at the visitor's center. The gun display will surely delight, and take the kids to the movie and cyclorama. The gift shop is very kid friendly.
The final half day take them to the eternal flame peace monument and visit the cannons. Also tell the story of the 11th PA and their mascot dog Sallie. The 11th PA monument is near the peace monument. Head over to Devil's Den, and let them climb around. Then head up to Little Round Top and the overlook tower there.
Have fun.