No offense but, are you sure you want to start your girls off shooting a shotgun? The kick and sound of a shotgun can be a little much for a small girl and if it bothers her you are fighting that forever.
I would start them with a cheap 22 rifle. Get them into shooting first, they can laugh, talk, move around and have fun. Later they can hunt rabbits and squirrels and have more fun. If rushed you might turn them away from hunting.