Dog was tied up outside corner shop - I could see him from the till point. Only picking up one item. Dog is usually friendly and calm. Child darts out of shop with father and runs towards my dog wanting to pet him. My dog reacts my growling and going to bite him. He didn’t thankfully but did hurt his lip by jumping. Dad is useless and just walks on to car. I quickly go outside and go to apologise. Mum who is sitting in the car is consoling child - I ask if he is hurt. She says just a scrape on mouth. I apologise again and explain that my dog must have been startled but I feel terrible and that I cannot trust my dog.