I would definitely try to write down a weekly schedule (or even monthly if you can) outlining the miles you drive and then showing how much in gas $ it really adds up to. Plus the wear and tear on your vehicle - how much sooner are you going to have to purchase another car due to the mileage you put on, how more frequently are you going to have to perform mileage maintenance and purchase tires?
If she and her husband are in the business world, she should understand these things. Plus the $ you are saving her cutting their hair, paying for parks - RIDICULOUS!! I agree w whoever posted to start bringing her receipts.
Unfortunately, I have found as long as you are willing to give, there is someone who is willing to keep taking.
I would keep the "talk" on an even, professional keel and just present facts. Hopefully she values how much you go above and beyond and will want to listen to your concerns bc she does not want to lose you for her children.