Thanks everyone for your replies!
I can see both sides here. You're all right in that as long as I'm getting paid on time, I don't really care what she's doing with that time. I know how important it is to carve out time from a busy schedule to fit in exercise!
I changed my contract for the new school year from opening and closing times to work and commute times only because I've been burned. I had one family that never picked up at the same time of day (it varied by as much as two hours) each day, and another who constantly pushed my closing time. We have a busy schedule during the school year with our children and their activities, and last year my contract stated that 5:30 was the latest pickup time allowable - this particular family would pick up five or ten minutes late on a regular basis, and we'd have to race to get places on time. Not only that, but I was giving them a hefty discount. It's because of that family (who won't be coming back!) that I'm hesitant to start conceding things right at the beginning.