Dog walking please answer?

Hello, im a 14 year old girl. Im very mature. I've had experience with dogs. I also have one of my own. I am interested in doing dog walking. I've waled my friends dogs before. I was just wondering if i could ask my friends mum if i could walk her jack russell. Do you think she will trust me? Her dog is a bit nippy ( he once bit my sister and she drew blood and had a mive bruise, i do not know why he hasn't been put down ) I have taken him for a walk before but only with his owner. Do you think she will let me take him by myself? i have made a leaflet explaining everything.