I would start by not leaving the pup alone with either of the children. I once saw a 6 month old golden retriever pup, jump up and bite a grown man in the face. He only needed one stitch, but can you imagine what can happen to a child. I would monitor all of the pups interactions with both children and correct any unwanted behavior. Also, maybe instead of telling the child to stay away from the dog, she can help with the care and training. Under strict supervision of course. This may help the pup to see her as leader and someone that he can't mess with.