C.S.
Is she doing stuff around your house for you? Currently I take my children to a sitter and pay 32 a day when they are UNDER 3, 29 a day if they are over three but not in full time school. Also, if you have two you get a $5 discount a day on one of them. She has set times, and anything over 4 hours is a full day. I wouldn't really do the gas thing either, the pay should be fair to cover things like that.. especially if she's in your home and will your food be helping feeding her kids? ( I assume she's not bringing something for HER kids, and then making your kids something different.. that usually doesn't work well). Also you have to figure out holidays and vacation pay... if any. Currently I give my sitter holidays, but no vacation. If she takes off, I don't pay. If I take off, I pay unless we have agreed otherwise.
Hope that helps?!