I have been an artist in residence at many schools and museums. Ruby-reuseable got it closest in her response.
I charge at least 50.00 and hour with a 4 hour min. Generally I also get paid for prep time also. Good artists/teachers spend HOURS on prep time, trust me.
Remember that we have to take 30% of that out for taxes that we pay ourselves plus we usually pay our own health insurance, etc. so $50.00 an hour isn't that much really. The artists with experience will want more and they are worth it generally.
I usually will just ask for a lump sum like $2000 for two weeks visiting 4 classes a day for three days a week with the promise that each child will create an original piece of work they can take home. I usually provide information/handouts for the teachers and provide all supplies. I try to make it easy on the school/teacher and bring EVERYTHING. This way I do not have to hand in receipts for supplies.
There are so many wonderful artists who would love to work in the classroom and share their knowledge and artists can always use the money to help them on their creative way, so I highly encourage you to do some research and find artist that LOVE working with kids.
I got a grant recently to have a well known artist come and work with some kids in a classroom last year, and when he came he was so uninterested and BORED with the whole thing, but he had been given a large grant from a museum to work with kids, so he was just showing up, but wasn't really into it.
Meet with the artist. See if he/she has some fire! Will inspire!
See samples of past work. It is money well spent if the artist is excited about their work and wants to share with your kids. Good luck!