Your "arrears" in your contribution will probably stand unless you settle with the other parent which would include a forgiveness of the arrears. Going forward, you must be able to persuade the court that there are changed circumstances or alternatives to the ordered daycare amount, as the court presumably came up with your paying 2/3 of the daycare based upon your income and her income and what the proportionate share should be.
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Short answer, no. All would have to be brought before a judge. Other parent would need to seek modification to add daycare cost - you should argue it is already covered within child support amount, in addition that you cannot afford it owing to what seems like an already high amount (percentage wise) of child support. Definitely bring up your alternative plan.
To questioners from West Virginia & New York: Although I am licensed to practice in your state, I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.
You can also argue that you shouldn't have to pay arrears for care when the custodial parent wasn't working or looking for work. And you can argue that you shouldn't have to pay for care that you never were presented a receipt for (if that is a provision in your order). Consult with a local attorney to figure out your best arguments based on the details of your orders.