My best friend just recently went through the same thing, the rate the mom could afford to pay her was just too little to make it worth while. It's a hard balance, because you don't want to see kids in poor quality care, but the reality is that $25 for two children is absurdly low. In our state the reimbursement rate for gov't subsidized care is $2.50 an hour for preschool aged children, even that in a 50 hour week would be $125 per week per child. That is the absolute lowest you will ever, ever find. I see pricing structures run two ways, either a per day rate, or a per week rate. At our center the per day rate lets you pay for less time, so we pay for 8 to 5, not 6:30 to 6:00 because we don't need the extra early/late care. The weekly rate allows you the whole 6:30 to 6 time frame at no extra fee. We pay about $40 a day, which is the high end of normal for this area, $150 to $200 a week for a licensed center, $125 or so for an unlicensed at home provider, is probably the norm.
If she can't find alternate care, perhaps you could offer phasing in the increase? At a rate that seems fair to you and she can handle, doubling her child care bill in two weeks is probably not feasible, I know for us it wouldn't be. Good luck! You definitely deserve more pay!