If you are considering this as a business transaction and not an emotional transaction, the act accordingly. This child has an outstanding balance of $___. I would mail a letter to both parents (exact same wording) along the lines of:
Date ______
Dear Mr. ____ & Ms. ______,
As of today July 19, 2011 you have a balance of _______ for services rendered on the following days ( ). This balance is due no later than _____. If you are unable to do so, I will no longer be able to care for your child until that balance is paid.
Thank you in advance for your cooperation.
Sincerely,
Rachel K.
Keep a copy for yourself and stick to it! If they bring you a check, cash or money order. Great. If they don't, then do not take care of the child. I know it's harsh, but the father is taking advantage of YOU and having a letter indicating his lack of responsibility will be relevant in court when the mother has to sue for additional child support OR when you sue for the balance.
Either way, this situation isn't a profitable one for you and if they were using a licensed provider they wouldn't be able to get away with this either!