S.S.
I would ask her first what she would charge for this. I myself would probably expect about $200 for this. I would not plan on the hubby might be home. Just plan 2 nights and go from there. She will be the person on call during school hours also yes? so if a kid gets sick and needs to be picked up I would assume she will be picking up. You may be surprised though as she may say oh how about "x" which will be less than you thought. But keep in mind if you think you might need this more often then offer a little more.