J.C.
I would pay her anywhere from $250-300 My reasoning for this is that the more you pay her, the more committed she will be do doing this for you.
As a teenager I did a ton of babysitting, I was even a nanny for one family. However, I usually would cancel the families that paid me less, to take on a family that would pay me more. Just the truth of the matter.
Paying her around $8 an hour seems reasonable and is above the min. wage.