We had ringworm go through out whole family, its very hard to get rid of too. Its not all that harmful thankfully, just itchy and ugly. Its a fungus. You can get it from anywhere, people, pets, outdoors, anywhere. If your dog had ringworm you would know it, it causes scaling and bald spots on animals. I had a litter of kittens that had it once. As for the dog, I would do a good check over her to see if she has any funny spots on her, or if she has been itching an unusual amount in any particular area. Its not that likely the baby got it from the dog, it could have been from anywhere.