Dog bite

I have a family dog that has never been aggressive. She plays with my 3 year old nephew and niece who also live here and cats and dogs. On 9/6 my girlfriends son who she played with and has been around for 3 days kept provoking her space. Just as a precaution we were strict on how he should play with the dog. So we always them supervised until they familiarize with each other. We want the kid to respect her space. However, in a split sec he got too comfy and fed her a cracker and laid his head on her face. She nipped him and ran away out of fear. Now the kid 35lbs and my dog 56lbs that nip caused a cut. I felt awful cause the kid didn't know what he was doing and my dog was just defending herself. We took the kid to the hospital I paid for his antibiotics, pain meds, car ride, cream, and offered to pay for the bill. I am now worried of being sued. The mom does not want to sue, but not sure if anyone else can sue me or take my dog please. I have more info too. Just ran out of room here.