If you have no interest in helping out this woman just politely tell her no thank you, you are not comfortable driving another child (or whatever the reason may be).
If you are interested (and I would be seeing as it's an easy way to make some extra money), you need to come up with a rate for each time you drive the child home.
She also must provide you with a car seat that can stay installed in your car. You do not want to have to install a car seat in your car every day. I have been there, done that before and it's just not worth the trouble. Also she should install it for you, you do not want to be at fault if you do not install it correctly.
I think the rate the pp suggested of $50/week would be fair. So $10/day every day there is school and you drive him home. I assume this would take less than a half hour, so you are basically earning $20/hour. I say go for it. Especially if the kids get along.