You did the right thing - it can be hard to speak up to your employer! It is ridiculous that she isn't even giving you gas money. You should definitely save all receipts (gas, admission costs, odds and ends that you pick up for her...) and total them for her weekly/biweekly/monthly. Give her a copy and keep one for yourself. These expenses are not yours - they are for HER kids. Hopefully she will be understanding.
The mileage rate is 51 cents/mile. This link is pretty interesting:
IRS Announces 2011 Standard Mileage Rates