Pay for 13 year old sitters?

13 year old is babysitting her 10 year old girl cousin for 7 - 10 hours three days a week. Grew up fairlly close. What is a fair pay? She is currently being paid $15/day, up from $10/day since Christmas vacation. 13 year old's mom says not enough. I (grammie to both) think it's fair. Answers?