A man on horseback in a thunderstorm is struck by lightning. Although he and the horse are thrown to the ground, the main effect of the strike is absorbed by the rifle he is carrying, which becomes twisted and inoperable.
The scenes on the European battlefield can be pretty intense, with bullets and hand-to-hand combat taking place, and characters dying. Despite this, very little blood is seen. If your kids have seen a modern PG-13 movie, they have seen far worse violence than they will see in Sergeant York.