C..
I paid my sitter (a college student) $75 to pick up my 10 year old from school at 3:30pm and stay overnight with her until she got on the bus the next morning at 7:30am.
I went out of town for a 4 days for a conference and I paid her $200.
My daughter was 10 and VERY self-suffient. But the girl was WORKING. She couldn't see her friends, boyfriend etc and I didn't want anyone coming over.
Not sure how old your kids are..... but being that there is more than 1, I would go with a couple hundred dollars at the minumum. If they are super young and she has to give them baths or change diapers and/or if they don't sleep through the night that number would need to be significantly higher, in my opinion.