Not a clue on how much to charge but...have you thought of some of these things?
1. What if your daughter (or if you have other kids, one of them) wants to do some type of after school activity when the other kids are there? Will you be able to drive your child to to/from the activity with the day care kids there (is your car big enough, is this other mom ok with that, etc.)?
If not, are you willing to sacrifice your kids after school activities for this?
I've already let her know that DD swims on Tues & Thurs and if the children aren't picked up before it's time to leave, they'll be coming with us and will need to be picked up at the pool. We have a Suburban so vehicle space isn't an issue.
2. Are you also the back up care in case school is cancelled for an emergency (might be a little early for snow...but flood, fire,etc. could happen that might cause the school to close down for a day or more)
Is that something you think you can do?
This is one thing we haven't yet worked out. The kids are out of school three days for parent/teacher conferences. She's looking to see if her husband will be able to work at home for those days. Other days like you mentioned, we'll need to discuss tomorrow!.
Just a few things I have thought of in the past when thinking about providing after school care...
Good luck to you!