How much is that stranger in the window?

Ugh. The spots are still with us. I went to work this morning (staff meeting, another meeting, a few thousand trips up and down the stairs) and when I got home at lunch, Hubby informed me the Boy Terror is now a spotted Terror, as well. So, we've narrowed the mystery down to "Contagious Mystery Spots".
We both have to work next week. It's considered very poor form for teachers to skip the first day of school. I was moaning to my friend about the lack of day care (nobody seems to want a house full of spotted kids. Imagine). She suggested I call her mother-in-law, who loves all children. I called Elaine. She would gladly help out, but she is working at a day care every day at 3, so no go. SHE gave me the number of her friend, Margaret. I don't know Margaret. I don't care. I called Margaret, and told her the entire story. She is happily coming over to watch the Terrors for at least two days next week, and hopefully by then the plague will have passed us by.
So here's my question. I have hired a complete stranger to come into my home and look after my children. What's the appropriate rate of pay for this? I know what she is worth to me, but I don't know how much money to offer her. What would you say is fair?