Going rate for younger baby sitters?

Hello, 
I have a 13 year old who will be offering baby sitting services and she wants to balance her rate to consider her age, but also be fair for the work and her time. It will be helpful for her to know what the normal range is for a younger sitter (say age 12, 13) that does have a lot of experience with babies and young children and is CPR certified. This would be in SF where the minimum wage is $18.67/hr. What are going rates for younger sitters for 1 child (and what are additional amounts for additional kids, and any age of child being cared for)? Thank you!