I would say $10 a day.
When speaking to her, just say, "Hey, thank you so much for offering to stop by every day while we are away, it is such a big help. I'll pay you $50 (assuming you will be gone 5 days) because I know it will take time out of your schedule for you to come over here." If she refuses, I would give her a gift card to a restaurant with a "thank you" card when we returned from vacation.