Here are some phrases: (I am adding a bit of behavior modification here...)
"Whenever you touch the dog, you must always be very gentle. Pet him like this." (Pet the dog gently in front of her)
"Now, YOU pet the dog." (Allow her to pet the dog gently, then say "Well done".)
From then on, when you see her petting the dog gently, smile and say "well done" or "very nice".
If she is being mean to him, stop her and remove the dog from her for a little bit. Tell her "if you are not nice to the dog, you can't pet him for now. We will try again later."
Repeat above when she is allowed to pet the dog again.
After a while, she will start saying, "look dad, see? I am being nice to the dog." It is a good idea to say something like " I see that - I like how you are so nice to the dog."
Hope that was helpful. :)