I'm right handed, left eye dominate. My right hand and arm are stronger and better for detail work, but my left hand and arm are more "steady" for lack of a better word. If I carry a shoulder bag it is always be on my left side, regardless of whether my strap is hanging off my left shoulder or across my back on the right. I'm a photographer by profession, and have always carried my camera bag on the left and mounted my camera to my left eye. My left hand and arm acts as the steady support for the camera and works the lens from below, while my right hand operates the camera itself.
Long guns I shoot lefty and sight with my left eye. I can shoot right handed okay and even sight with my right eye okay, left just feels more natural and the way I've been doing it since I was a little kid with a pop gun. I shoot pistol primarily right handed, but practice both, mostly sighted with the left eye. Shooting with both eyes open is no bueno for me.
I know I could train my brain to use my right eye, but see no reason to. My body adapted and learned to do certain things left handed way back when I was a kid.
You might suggest to your granddaughter she try shooting left handed and see which she prefers. So long as she's not putting her face directly behind the rifle where she could get hurt, let her do whatever she is comfortable with so long as she is having fun. I've got older lefty friends who tell real horror stories about being forced to do things right handed as a child back when we were stupid enough to think left handed was somehow wrong. I assume it would be the same in reverse. So safety should be your only priority over having fun.