Hi OP. Pay sounds good for the amount of hours. If he provides the car, no need to pay mileage, but of course he would be required to keep car gassed up.
I agree with finding someone with experience with this age group (working with teens is totally different than babies/toddlers) and agree that she should be 10+ years older than the girls, this should automatically give her some authority.
I've been working in a similar position for 3 years now, the kids are 16, 14 (twins), & 7. Same hours, pay is a bit less. I use my car and the family pays me mileage. Besides caring for the kids and driving, I also do errands, light housekeeping, organizing, light cooking, etc.
Would be happy to answer any questions you have.