She sounds like the unreasonable one! You have been more than reasonable to her! A christmas bonus plus a weeks pay for not having to do anything. I'd be very happy with that! She sounds like shes taking you for a ride. Why would she expect to be paid for a day she doesn't have the kids. It doesn't make sense. You work around her all the time.

I'd have a serious chat with her about it. She doesn't need to be spending any money on your kids and the food you provide is well and truly more than enough for 2 kids.