T.F.
This does depend on your location. I am in a northern burb of Dallas, TX and my 16 yr old averages $12-$15/hour with 3 regular families she babysits for.
Parents are not home when she is there, she is the primary caregiver until mom/dad get home including, feeding, diapering, playing with and and making sure any messes are cleaned up. The little girls love her because she does manis/pedis with little gems (with mom's approval of course).
The "average" here is arund $10 cash per hour.
You say you are there and she is helping you? When my daughter acted as mom's helped at 13 yrs old, she got $5-$7 per hour cash.